# A Three-Level Sieve Algorithm for the Shortest Vector Problem

• Feng Zhang
• Yanbin Pan
• Gengran Hu
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 8282)

## Abstract

In AsiaCCS 2011, Wang et al. proposed a two-level heuristic sieve algorithm for the shortest vector problem in lattices, which improves the Nguyen-Vidick sieve algorithm. Inspired by their idea, we present a three-level sieve algorithm in this paper, which is shown to have better time complexity. More precisely, the time complexity of our algorithm is $$2^{0.3778n+o(n)}$$ polynomial-time operations and the corresponding space complexity is $$2^{0.2833n+o(n)}$$ polynomially many bits.

## Keywords

Lattice Shortest vector problem Sieve algorithm Sphere covering

## 1 Introduction

Lattices are discrete subgroups of $$\mathbb {R}^n$$ and have been widely used in cryptology. The shortest vector problem(SVP) refers the question to find a shortest non-zero vector in a given lattice, which is one of the most famous and widely studied computational problems on lattices.

It is well known that SVP is NP-hard under random reductions [2], so no polynomial time exact algorithms for it are expected to exist. Up to now, only approximation algorithms, such as [7, 8, 13, 25], are efficient and all known exact algorithms are proven to cost exponential time. However, almost all known approximation algorithms (such as [8, 25]) invoke some exact algorithm for solving SVP on some low dimensional lattices to improve the quantity of their outputs. Therefore, it is important to know how fast the best exact algorithm can be. What’s more, algorithms for SVP play a very important role in cryptanalysis (see [19] for a survey). For example, nearly all knapsack-based public-key cryptosystems have been broken with a lattice algorithm (see [1, 14, 27]) and many lattice-based public-key cryptosystems can be broken by solving some SVP, including the famous NTRU [10]. Hence, better exact algorithm for SVP can also help us to know the security of these lattice-based public-key cryptosystems better, and choose more appropriate parameters for these cryptosystems.

The exact algorithms for SVP can be classified into two classes by now: deterministic algorithms and randomized sieve algorithms.

The first deterministic algorithm to find the shortest vector in a given lattice was proposed by Fincke, Pohst [5, 6] and Kannan [11], by enumerating all lattice vectors shorter than a prescribed bound. If the input is an LLL-reduced basis, the running time is $$2^{O(n^2)}$$ polynomial-time operations. Kannan [11] also showed the running time can reach $$2^{O(nlog n)}$$ polynomial-time operations by choosing a suitable preprocessing algorithm. Schnorr and Euchner [26] presented a zig-zag strategy for enumerating the lattice vectors to make the algorithm have a better performance in practice. In 2010, Gama, Nguyen and Regev [9] introduced an extreme pruning technique and improved the running time in both theory and practice. All enumeration algorithms above require a polynomial space complexity. Another deterministic algorithm for SVP was proposed by Micciancio and Voulgaris [15] in 2010. Different from the previous algorithms, it is based on Voronoi cell computation and is the first deterministic single exponential time exact algorithm for SVP. The time complexity is $$2^{2n+o(n)}$$ polynomial-time operations. One drawback of the algorithm is that its space requirement is not polynomial but $$2^{O(n)}$$.

The randomized sieve algorithm was discovered by Ajtai, Kumar and Sivakumar (AKS) [3] in 2001. The running time and space requirement were proven to be $$2^{O(n)}$$. Regev’s alternative analysis [22] showed that the hidden constant in $$O(n)$$ was at most 16, and it was further decreased to 5.9 by Nguyen and Vidick [20]. Blömer and Naewe [4] generalized the results of AKS to $$l_p$$ norms. Micciancio and Voulgaris [16] presented a provable sieving variant called the ListSieve algorithm, whose running time is $$2^{3.199n+o(n)}$$ polynomial-time operations and space requirement is $$2^{1.325n+o(n)}$$ polynomially many bits. Subsequently, Pujol and Stehlé [21] improved the theoretical bound of the ListSieve algorithm to running time $$2^{2.465n+o(n)}$$ and space $$2^{1.233n+o(n)}$$ by introducing the birthday attack strategy. In the same work [16], Micciancio and Voulgaris also presented a heuristic variant of the ListSieve algorithm, called the GaussSieve algorithm. However, no upper bound on the running time of the GaussSieve Algorithm is currently known and the space requirement is provably bounded by $$2^{0.41n}$$. In [23], Schneider analyzed the GaussSieve algorithm and showed its strengths and weakness. What’s more, a parallel implementation of the GaussSieve algorithm was presented by Milde and Schneider [17]. Recently, Schneider [24] presented an IdealListSieve algorithm to improve the ListSieve algorithm for the shortest vector problem in ideal lattices and the practical speed up is linear in the degree of the field polynomial. He also proposed a variant of the heuristic GaussSieve algorithm for ideal lattice with the same speedup.

To give a correct analysis of its complexity, the AKS algorithm involves some perturbations. However, getting rid of the perturbations, Nguyen and Vidick [20] proposed the first heuristic variant of the AKS algorithm, which in practice performs better and can solve SVP up to dimension 50. Its running time was proven to be $$2^{0.415n+o(n)}$$ polynomial-time operations under some nature heuristic assumption of uniform distribution of the sieved lattice vectors. By introducing a two-level technique, Wang et al. [30] gave an algorithm (WLTB) to improve the Nguyen-Vidick algorithm. Under a similar assumption of the distribution of sieved lattice vectors, the WLTB algorithm has the best theoretical time complexity so far, that is, $$2^{0.3836n+o(n)}$$. Both the heuristic assumptions can be supported by the experimental results on low dimensional lattices.

Our Contribution. Observing that the WLTB algorithm involves some data structure like skip list to reduce the time complexity, we present a three-level sieve algorithm in this paper. To estimate the complexity of the algorithm, it needs to compute the volume of some irregular spherical cap, which is a very complicated and tough work. By involving a smart technique, we simplify the complicated computation and prove that the optimal time complexity is $$2^{0.3778n+o(n)}$$ polynomial-time operations and the corresponding space complexity is $$2^{0.2833n+o(n)}$$ polynomially many bits under a similar natural heuristic assumption.

Table 1 summarizes the complexities of the heuristic variants of AKS algorithm and the GaussSieve algorithm. It can be seen that the latter two algorithms employ the time-memory tradeoffs that decrease the running time complexity at the cost of space complexity.
Table 1.

Complexities of some heuristic algorithms for SVP

Algorithm

Time complexity

Space complexity

GaussSieve algorithm

-

$$2^{0.41n+o(n)}$$

Nguyen-Vidick algorithm

$$2^{0.415n+o(n)}$$

$$2^{0.2075n+o(n)}$$

WLTB algorithm

$$2^{0.3836n+o(n)}$$

$$2^{0.2557n+o(n)}$$

Our three-level algorithm

$$2^{0.3778n+o(n)}$$

$$2^{0.2883n+o(n)}$$

A natural question is whether we can improve the time complexity by four-level or higher-level algorithm. It may have a positive answer. However, by our work, it seems that the improvements get smaller and smaller, whereas the analysis of the complexity becomes more and more difficult when the number of levels increases.

Road Map. The rest of the paper is organized as follows. In Sect. 2 we provide some notations and preliminaries. We present our three-level sieve algorithm and the detailed analysis of its complexity in Sect. 3. Some experimental results are described in Sect. 4. Finally, Sect. 5 gives a short conclusion.

## 2 Notations and Preliminaries

Notations. Bold lower-case letters are used to denote vectors in $$\mathbb {R}^n$$. Denote by $$v_i$$ the $$i$$-th entry of a vector $$\varvec{v}$$. Let $$\Vert \cdot \Vert$$ and $$\langle \cdot ,\cdot \rangle$$ be the Euclidean norm and inner product of $$\mathbb {R}^n$$. Matrices are written as bold capital letters and the $$i$$-th column vector of a matrix $$\varvec{B}$$ is denoted by $$\varvec{b}_i$$.

Let $$B_n(\varvec{x},R)=\{\varvec{y}\in \mathbb {R}^n\;|\;\Vert \varvec{y}-\varvec{x}\Vert \le R\}$$ be the $$n$$-dimensional ball centered at $$\varvec{x}$$ with radius $$R$$. Let $$B_n(R)=B_n(\mathbf {O},R)$$. Let $$C_n(\gamma , R)=\{\varvec{x}\in \mathbb {R}^n\;|\;\gamma R\le \Vert \varvec{x}\Vert \le R\}$$ be a spherical shell in $$B_n(R)$$, and $$S^n=\{\varvec{x}\in \mathbb {R}^n\;|\;\Vert \varvec{x}\Vert =1\}$$ be the unit sphere in $$\mathbb {R}^n$$. Denote by $$|S^n|$$ the area of $$S^n.$$

### 2.1 Lattices

Let $$\varvec{B}=\{{\varvec{b}_1,\varvec{b}_2,\ldots ,\varvec{b}_n}\}\subset \mathbb {R}^{m}$$ be a set of $$n$$ linearly independent vectors. The lattice $$\mathcal {L}$$ generated by the basis $$\varvec{B}$$ is defined as $$\mathcal {L}(\varvec{B})=\left\{ \sum _{i=1}^{n}x_i\mathbf b _i:x_i\in \mathbb {Z}\right\} .$$ $$n$$ is called the rank of the lattice. Denote by $$\lambda _1(\mathcal {L})$$ the norm of a shortest non-zero vector of $$\mathcal {L}$$.

### 2.2 The Basic Framework of Some Heuristic Sieve Algorithms

The Nguyen-Vidick algorithm and the WLTB algorithm have a common basic framework, which can be described as Algorithm 1 [30].

In general the Sieve Algorithm in Line 8 will output a set $$S'$$ of shorter lattice vectors than those in $$S$$. When we repeat the sieve process enough times, a shortest vector is expected to be found.

Denote by $$R'$$ (resp. $$R$$) the maximum length of those vectors in $$S'$$ (resp. $$S$$). To find $$S'$$, the sieve algorithm usually tries to find a set $$C$$ of lattice vectors in $$S$$ such that the balls centered at these vectors with radius $$R'$$ can cover all the lattice points in some spherical shell $$C_n(\gamma , R)$$. By subtracting the corresponding center from every lattice point in every ball, shorter lattice vectors will be obtained, which form the set $$S'$$.

Different ways to find $$C$$ lead to different algorithms. Roughly speaking,
• The Nguyen-Vidick algorithm checks every lattice point in $$S'$$ sequentially to decide whether it is also in some existing ball or it is a new vector in $$C$$ (see Fig. 1 for a geometric description).

• The WLTB algorithm involves a two-level strategy, that is, the big-ball-level and the small-ball-level. It first covers the spherical shell with big balls centered at some lattice vectors, then covers the intersection of every big ball and $$C_n(\gamma , R)$$ with small balls centered at some lattice points in the intersection. The centers of the small balls form $$C$$. It can be shown that it is faster to decide whether a lattice vector is in $$C$$ or not. We first check whether it is in some big ball or not. If not, it must be a new point in $$C$$. If so, we just check whether it is in some small ball in the big ball it belongs to, regardless of those small balls of the other big balls (see Fig. 2 for a geometric description).

For either the Nguyen-Vidick algorithm or the WLTB algorithm, to analyze its complexity needs a natural assumption below.

Heuristic Assumption 1: At any stage in Algorithm 1, the lattice vectors in $$S'\cap C_n(\gamma , R)$$ are uniformly distributed in $$C_n(\gamma , R)$$.

## 3 A Three-Level Sieve Algorithm

### 3.1 Description of the Three-Level Sieve Algorithm

Different from the two-level algorithm, our algorithm involves a medium-ball-level. Simply speaking, the algorithm first covers the spherical shell with big balls, then covers every big ball with medium balls, and at last covers every medium ball with small balls. Algorithm 2 gives a detailed description of the three-level sieve algorithm. In Algorithm 2, $$0.88<\gamma _3<1<\gamma _{2}<\gamma _1<\sqrt{2}\gamma _3$$. The set $$C_1$$ is the collection of centers of big balls with radius $$\gamma _1R$$ in the first level. For any $$\varvec{c}_1\in C_1$$, $$C_2^{\varvec{c}_1}$$ is the set of centers of medium balls with radius $$\gamma _2R$$ that cover the big spherical cap $$B_n(\varvec{c}_1,\gamma _1R)\cap C_n(\gamma _3,R).$$ It is clear that the elements of $$C_2^{\varvec{c}_1}$$ are chosen from $$B_n(\varvec{c}_1,\gamma _1R)\cap C_n(\gamma _3,R).$$ For $$\varvec{c}_1\in C_1, \varvec{c}_2\in C_2^{\varvec{c}_1}$$, $$C_3^{\varvec{c}_1,\varvec{c}_2}$$ is the set of centers of small balls with radius $$\gamma _3R$$ that cover the small spherical cap $$B_n(\varvec{c}_2,\gamma _2R)\cap B_n(\varvec{c}_1,\gamma _1R)\cap C_n(\gamma _3,R).$$ Also the elements of $$C_3^{\varvec{c}_1,\varvec{c}_2}$$ are chosen from the small spherical cap.

### 3.2 Complexity of the Algorithm

Denote by $$N_1$$, $$N_2$$ and $$N_3$$ the corresponding upper bound on the expected number of lattice points in $$C_1$$, $$C_2^{\varvec{c}_1}$$ (for any $$\varvec{c}_1\in C_1$$) and $$C_3^{\varvec{c}_1,\varvec{c}_2}$$ (for any $$\varvec{c}_1\in C_1,\varvec{c}_2\in C_2^{\varvec{c}_1}$$).

The Space Complexity. Notice that the total number of big, medium and small balls can be bounded by $$N_1$$, $$N_1N_2$$ and $$N_1N_2N_3$$ respectively. As in [20] and [30], if we sample $$\mathrm {poly}(n)N_1N_2N_3$$ vectors, after a polynomial number of iterations in Algorithm 1, it is expected that a shortest non-zero lattice vector can be obtained with the left vectors. So the space complexity is bounded by $$O(N_1N_2N_3)$$.

The Time Complexity. The initial size of $$S$$ is $$\mathrm {poly}(n)N_1N_2N_3$$. In each iteration in Algorithm 1, steps 3–19 in Algorithm 2 repeat $$\mathrm {poly}(n)N_1N_2N_3$$ times, and in each repeat, at most $$N_1+N_2+N_3$$ comparisons are needed. Therefore, the total time complexity can be bounded by $$O(N_1N_2N_3(N_1+N_2+N_3))$$ polynomial-time operations.

We next give the estimation of $$N_1$$, $$N_2$$ and $$N_3$$. Without loss of generality, we restrict $$R=1$$ and let $$C_n(\gamma )=C_n(\gamma ,1)=\{\varvec{x}\in \mathbb {R}^n\;|\;\gamma R\le \Vert \varvec{x}\Vert \le 1\}$$ through our proofs for simplicity.

The Upper Bound of $$N_1.$$ Nguyen and Vidick [20] first gave a proof of the upper bound $$N_1$$, and a more refined proof was given by Wang et al. [30].

### Theorem 1

(Wang et al. [30]) Let $$n$$ be a non-negative integer, $$N$$ be an integer and $$0.88<\gamma _3<1<\gamma _1<\sqrt{2}\gamma _3$$. Let
$$N_{1}=c_{\mathcal {H}_{1}}^n\lceil 3\sqrt{2\pi }n^{\frac{3}{2}}\rceil ,$$
where $$c_{\mathcal {H}_1}=1/(\gamma _1\sqrt{1-\frac{\gamma _1^2}{4}})$$ and $$S$$ a subset of $$C_n(\gamma _3 R)$$ of cardinality $$N$$ whose points are picked independently at random with uniform distribution. If $$N_{1}<N<2^n$$, then for any subset $$C\subseteq S$$ of size at least $$N_{1}$$ whose points are picked independently at random with uniform distribution, with overwhelming probability, for all $$\varvec{v}\in S$$, there exists a $$\varvec{c}\in C$$ such that $$\Vert \varvec{v}-\varvec{c}\Vert \le \gamma _1 R$$.
The Upper Bound of $$N_2.$$ Let
• $$\varOmega _n(\gamma _1)$$ be the fraction of $$C_n(\gamma _3)$$ that is covered by a ball of radius $$\gamma _1$$ centered at a point of $$C_n(\gamma _3)$$,

• $$\varGamma _n(\gamma _1,\gamma _2)$$ be the fraction of $$C_n(\gamma _3)$$ covered by a big spherical cap $$B_n(\varvec{c}_2,\gamma _2)\cap B_n(\varvec{c}_1,\gamma _1)\cap$$ $$C_n(\gamma _3)$$,

• $$\varOmega _n(\gamma _1,\gamma _2)$$ be the fraction of $$B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ covered by $$B_n(\varvec{c}_2,\gamma _2)\cap B_n(\varvec{c}_1,\gamma _1)$$ $$\cap C_n(\gamma _3)$$, where $$\varvec{c}_2\in C_2^{\varvec{c}_1},\varvec{c}_1\in C_1$$.

Clearly, $$\varOmega _n(\gamma _1,\gamma _2)=\frac{\varGamma _n(\gamma _1,\gamma _2)}{\varOmega _n(\gamma _1)}.$$ To compute $$N_2$$, we need the minimal value of $$\varOmega _n(\gamma _1,\gamma _2)$$. We estimate $$\varOmega _n(\gamma _1)$$ and $$\varGamma _n(\gamma _1,\gamma _2)$$ respectively.

### Lemma 1

(Wang et al. [30]) Let $$0.88<\gamma _3<1<\gamma _1<\sqrt{2}\gamma _3,$$ then
$$\frac{1}{3\sqrt{2\pi n}}\frac{(\sin \theta _2)^{n-1}}{\cos \theta _2} <\varOmega _n(\gamma _1)<\frac{1}{\sqrt{2\pi (n-1)}}\frac{(\sin \theta _1)^{n-1}}{\cos \theta _1},$$
where $$\theta _1=\arccos (1-\frac{\gamma _1^2}{2\gamma _3^2}), \theta _2=\arccos (1-\frac{\gamma _1^2}{2}).$$

Note that the proportion $$\varGamma _n(\gamma _1,\gamma _2)$$ is different from that of Lemma 4 in [30], as the radius of $$B_n(\varvec{c}_2,\gamma _2)$$ is larger than the inside radius of the shell $$C_n(\gamma _3)$$. Thus, it leads to the slightly different bounds of $$\varGamma _n(\gamma _1,\gamma _2)$$ from that of Lemma 4 in [30]. If $$\varvec{c}_2$$ lies on the sphere of a big ball $$B_n(\varvec{c}_1,\gamma _1)$$, the fraction $$\varGamma _n(\gamma _1,\gamma _2)$$ is minimal. Lemma 2 gives the minimal and maximal value of $$\varGamma _n(\gamma _1,\gamma _2)$$ when $$\varvec{c}_2$$ lies on the sphere of a big ball $$B_n(\varvec{c}_1,\gamma _1)$$.

### Lemma 2

Let $$0.88<\gamma _3<1<\gamma _2<\gamma _1<\sqrt{2}\gamma _3$$, where $$\gamma _3$$ is very close to $$1$$. Then
$$\frac{cd_{\min }^{n-2}}{{2\pi n}}\le \varGamma _n(\gamma _1,\gamma _2)\le \frac{c'd_{\max }^{n-2}}{{2\pi }},$$
where $$d_{\max }=\sqrt{1-\left( \frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) ^2- \left( \frac{1}{c_{\mathcal {H}_2}}\left( \frac{\gamma _3^2+1-\gamma _2^2}{2}- \frac{(2\gamma _3^2-\gamma _1^2)(\gamma _3^2-\gamma _1^2+1)}{4\gamma _3^2}\right) \right) ^2},$$ $$d_{\min } =\gamma _2\sqrt{1-\frac{\gamma _2^2c_{\mathcal {H}_{1}}^2}{4}},\;c_{\mathcal {H}_1}=1/(\gamma _1\sqrt{1-\frac{\gamma _1^2}{4}}),\;c_{\mathcal {H}_2}=\frac{\gamma _{1}}{\gamma _3}\sqrt{1-\frac{\gamma _{1}^2}{4\gamma _3^2}},$$ $$c$$ and $$c'$$ are constants unrelated to $$n$$.

### Proof

Note that $$\gamma _3$$ is very close to $$1$$. We just consider the proportion on the sphere covering as in [30].

Without loss of generality, we assume the center of $$B_n(\varvec{c}_1,\gamma _1)$$ is $$\varvec{c}_1= (\alpha _1,0,\ldots ,0)$$, and the center of $$B_n(\varvec{c}_2,\gamma _2)$$ is $$\varvec{c}_2=(\beta _1,\beta _2,0\ldots ,0)$$, where $$\alpha ,\beta _1,\beta _2 >0$$. The spherical cap $$B_n(\varvec{c}_2,\gamma _2) \cap B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ is
$$\left\{ \begin{array}{l} x_1^2+x_2^2+\ldots +x_n^2=1\\ (x_1-\alpha _1)^2+x_2^2+\ldots +x_n^2<\gamma _1^2\\ (x_1-\beta _1)^2+(x_2-\beta _2)^2+\ldots +x_n^2<\gamma _2^2 \end{array}\right.$$
where $$\gamma _3\le \alpha _1\le 1, (\beta _1-\alpha _1)^2+\beta _2^2=\gamma _1^2$$ and $$\gamma _3^2\le \beta _1^2+\beta _2^2\le 1$$. The region is as the shadow of the Fig. 3. Denote by $$Q$$ the volume of the region. By projecting the target region to the hyperplane orthogonal to $$x_1$$ and by sphere coordinate transformation (for details see the proof of Lemma 4 in [30]), we get
$$\frac{c d^{n-2}}{2\pi n}\le \varGamma _n(\gamma _{1},\gamma _2)=\frac{Q}{|S^n|}\le \frac{c' d^{n-2}}{2\pi }$$
where $$d=\sqrt{1-\left( \frac{\alpha _1^2-\gamma _1^2+1}{2\alpha _1}\right) ^2-\left( \frac{1}{\beta _2}\left( \frac{\beta _1^2+\beta _2^2+1-\gamma _2^2}{2}-\beta _1\frac{\alpha _1^2-\gamma _1^2+1}{2\alpha _1}\right) \right) ^2}$$ and $$c,c'$$ are constants unrelated to $$n$$. Let $$\alpha _2=\sqrt{\beta _1^2+\beta _2^2}$$. From the equation $$(\beta _1-\alpha _1)^2+\beta _2^2=\gamma _1^2$$, we obtain
$$\beta _1=\frac{\alpha _2^2+\alpha _1^2-\gamma _1^2}{2\alpha _1}, \beta _2=\sqrt{\alpha _2^2-\left( \frac{\alpha _2^2+\alpha _1^2-\gamma _1^2}{2\alpha _1}\right) ^2}.$$
Therefore, $$d$$ can be regarded as a function with respect to $$\alpha _1,\alpha _2$$, where $$\gamma _3\le \alpha _1\le 1, \gamma _3\le \alpha _2\le 1$$. Since $$0.88<\gamma _3<1<\gamma _2<\gamma _1<\sqrt{2}\gamma _3$$, it can be proven that $$d$$ decreases with $$\alpha _1,\alpha _2$$ increasing. Then $$d_{\min }$$ can be obtained by letting $$\alpha _1=1,\alpha _2=1$$ and $$d_{\max }$$ can be obtained by letting $$\alpha _1=\gamma _3,\alpha _2=\gamma _3$$. Hence, the lemma follows.

### Theorem 2

Let $$n$$ be a non-negative integer, $$N$$ be an integer and $$0.88<\gamma _3<1<\gamma _2<\gamma _1<\sqrt{2}\gamma _3$$, where $$\gamma _3$$ is very close to 1. Let
$$N_{2}=c_2(\frac{c_{\mathcal {H}_2}}{d_{\min }})^n\lceil n^{\frac{3}{2}}\rceil ,$$
where $$c_{\mathcal {H}_2}=\frac{\gamma _{1}}{\gamma _3}\sqrt{1-\frac{\gamma _{1}^2}{4\gamma _3^2}}, d_{\min }=\gamma _2\sqrt{1-\frac{\gamma _2^2c_{\mathcal {H}_{1}}^2}{4}},c_{\mathcal {H}_1}=1/(\gamma _1\sqrt{1-\frac{\gamma _1^2}{4}})$$, and $$c_2$$ is a positive constant unrelated to $$n$$. Let $$S$$ be a subset of $$C_n(\gamma _3 R)\cap B_n(\varvec{c}_{1},\gamma _{1} R)\cap B_n(\varvec{c}_2,\gamma _2 R)$$ of cardinality $$N$$ whose points are picked independently at random with uniform distribution. If $$N_{2}<N<2^n$$, then for any subset $$C\subseteq S$$ of size at least $$N_{2}$$ whose points are picked independently at random with uniform distribution, with overwhelming probability, for all $$\varvec{v}\in S$$, there exists a $$\varvec{c}\in C$$ such that $$\Vert \varvec{v}-\varvec{c}\Vert \le \gamma _2 R$$.

### Proof

Combining Lemmas 1 and 2, we have $$\varOmega _n(\gamma _1,\gamma _2)=\frac{\varGamma _n(\gamma _1,\gamma _2)}{\varOmega _n(\gamma _1)}\ge \frac{c}{\sqrt{2\pi n}}\cdot \left( 1-\frac{\gamma _1^2}{2\gamma _2^2}\right) \left( \frac{d_{\min }}{c_{\mathcal {H}_2}}\right) ^n.$$ The expected fraction of $$B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ that is not covered by $$N_{2}$$ balls of radius $$\gamma _2$$ centered at randomly chosen points of $$B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ is $$(1-\varOmega _n(\gamma _1,\gamma _2))^{N_{2}}$$. So,
\begin{aligned} N_{2}\log (1-\varOmega _n(\gamma _1,\gamma _2))&\le N_{2}(-\varOmega _n(\gamma _1,\gamma _2))\\&<c_2 n^{3/2}\left( \frac{c_{\mathcal {H}_2}}{d_{\min }}\right) ^n\cdot \frac{1}{c_2\sqrt{n}}\left( \frac{d_{\min }}{c_{\mathcal {H}_2}}\right) ^n \le -n<-\log N. \end{aligned}
which implies $$(1-\varOmega _n(\gamma _1,\gamma _2))^{N_{2}}<e^{-n}<\frac{1}{N}.$$ The expected number of uncovered points is smaller than 1. It means that any point in $$B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ is covered by a ball centered at a vector in $$B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ with radius $$\gamma _2$$ with probability $$1-e^{-n}$$.
The Upper Bound of $$N_3.$$ Let
• $$\varGamma _n(\gamma _1,\gamma _2,\gamma _3)$$ be the fraction of $$C_n(\gamma _3)$$ that is covered by a small spherical cap $$B_n(\varvec{c}_3,\gamma _3)\cap B_n(\varvec{c}_2,\gamma _2)\cap$$ $$B_n(\varvec{c}_1,\gamma _1)\cap$$ $$C_n(\gamma _3)$$,

• $$\varOmega _n(\gamma _1,\gamma _2,\gamma _3)$$ the fraction of $$B_n(\varvec{c}_2,\gamma _2)\cap B_n(\varvec{c}_1,\gamma _1)$$ $$\cap C_n(\gamma _3)$$ covered by $$B_n(\varvec{c}_3,\gamma _3)$$ $$\cap B_n(\varvec{c}_2,\gamma _2)\cap B_n(\varvec{c}_1,\gamma _1)\cap$$ $$C_n(\gamma _3)$$, where $$\varvec{c}_3\in C_3^{\varvec{c}_1,\varvec{c}_2},\varvec{c}_2\in C_2^{\varvec{c}_1},\varvec{c}_1\in C_1.$$

Clearly, $$\varOmega _n(\gamma _1,\gamma _2,\gamma _3) =\frac{\varGamma _n(\gamma _1,\gamma _2,\gamma _3)}{\varGamma _n(\gamma _1,\gamma _2)}.$$ To estimate $$N_3$$, we need to compute the lower bound of $$\varOmega _n(\gamma _1,\gamma _2,\gamma _3)$$. To obtain the lower bound of $$\varGamma _n(\gamma _1,\gamma _2,\gamma _3)$$, we need to compute the volume of some irregular convex region, which is very complicated. However, using the inscribed triangle of the region, we get a reasonable lower bound of the volume successfully.

### Lemma 3

Let $$0.88<\gamma _3<1<\gamma _2<\gamma _1<\sqrt{2}\gamma _3$$, where $$\gamma _3$$ is very close to $$1$$. We have
$$\varGamma _n(\gamma _1,\gamma _2,\gamma _3)\ge \frac{c''r_{\min }^{n-3}}{2\pi ^{3/2}n^2},$$
where $$r_{\min }=\sqrt{c_{\mathcal {H}_3}-\left( 1-\frac{\gamma _3^2}{2c_{\mathcal {H}_3}}\right) ^2}$$, $$c_{\mathcal {H}_3}=\gamma _2^2\left( 1-\frac{\gamma _2^2c_{\mathcal {H}_{1}}^2}{4}\right) ,c''$$ is a constant unrelated to $$n$$.

### Proof

We consider the proportion on the sphere covering. W.l.o.g., we assume the centers of $$B_n(\varvec{c}_1,\gamma _1),B_n(\varvec{c}_2,\gamma _2),$$ $$B_n(\varvec{c}_3,\gamma _3)$$ are, respectively,
\begin{aligned} \varvec{c}_1&=(\alpha _1,0,\ldots ,0),\;\alpha _1 > 0,\\ \varvec{c}_2&=(\beta _1,\beta _2,0\ldots ,0), \;\beta _1,\beta _2>0,\\ \varvec{c}_3&=(\delta _1,\delta _2,\delta _3,0\ldots ,0),\; \delta _1,\delta _2,\delta _3>0. \end{aligned}
The spherical cap $$B_n(\varvec{c}_3,\gamma _3)\cap B_n(\varvec{c}_2,\gamma _2)\cap B_n(\varvec{c}_1,\gamma _1)\cap C_n(\gamma _3)$$ is
$$\left\{ \begin{array}{l@{\quad }l} x_1^2+x_2^2+\ldots +x_n^2=1&{}(E_1)\\ (x_1-\alpha _1)^2+x_2^2+\ldots +x_n^2<\gamma _1^2&{}(E_2)\\ (x_1-\beta _1)^2+(x_2-\beta _2)^2+x_3^2+\ldots +x_n^2<\gamma _2^2&{}(E_3)\\ (x_1-\delta _1)^2+(x_2-\delta _2)^2+(x_3-\delta _3)^2+\ldots +x_n^2<\gamma _3^2&{}(E_4) \end{array}\right.$$
where $$\gamma _3\le \alpha _1\le 1,\gamma _3^2\le \beta _1^2+\beta _2^2\le 1, (\beta _1-\alpha _1)^2+\beta _2^2=\gamma _1^2,\gamma _3^2\le \delta _1^2+\delta _2^2+\delta _3^2\le 1, (\delta _1-\alpha _1)^2+\delta _2^2+\delta _3^2=\gamma _1^2,(\delta _1-\beta _1)^2+(\delta _2-\beta _2)^2+\delta _3^2=\gamma _2^2$$.
Denote by $$Q$$ the volume of the region, and project the target region to the hyperplane orthogonal to $$x_1.$$ Denote by $$D$$ the projection region. Therefore, the volume of the target region is
$$Q=\int \int \cdots \int _D\sqrt{1+\sum _{i=2}^{n}\left( \frac{\partial x_1}{\partial x_i}\right) ^2}\text {d}x_2\text {d}x_3\cdots \text {d}x_n =\int \int \cdots \int _D\frac{\text {d}x_2\text {d}x_3\cdots \text {d}x_n}{\sqrt{1-\sum _{i=2}^{n}x_i^2}}.$$
Now we determine the projection region $$D$$. To simplify the expression, we let $$\alpha _2=\sqrt{\beta _1^2+\beta _2^2}, \alpha _3=\sqrt{\delta _1^2+\delta _2^2+\delta _3^2}, a=\frac{\alpha _1^2+1-\gamma _1^2}{2\alpha _1}, b=\frac{\alpha _2^2+1-\gamma _2^2}{2}, f=\frac{\alpha _3^2+1-\gamma _3^2}{2}.$$ From the equations $$(\beta _1-\alpha _1)^2+\beta _2^2=\gamma _1^2, (\delta _1-\alpha _1)^2+\delta _2^2+\delta _3^2=\gamma _1^2,(\delta _1-\beta _1)^2+(\delta _2-\beta _2)^2+\delta _3^2=\gamma _2^2$$, it is easy to write $$\beta _1,\beta _2,\delta _1,\delta _2,\delta _3$$ as the expressions of $$\alpha _i,\gamma _i,i=1,2,3,$$ i.e.,
\begin{aligned} \beta _1&=\frac{\alpha _1^2+\alpha _2^2-\gamma _1^2}{2\alpha _1}, \quad \beta _2=\sqrt{\alpha _2^2-\left( \frac{\alpha _2^2+\alpha _1^2-\gamma _1^2}{2\alpha _1}\right) ^2},\\ \delta _1&=\frac{\alpha _1^2+\alpha _3^2-\gamma _1^2}{2\alpha _1},\quad \delta _2=\frac{\alpha _2^2+\alpha _3^2-\gamma _2^2-\frac{\left( \alpha _1^2+\alpha _2^2-\gamma _1^2\right) \left( \alpha _1^2+\alpha _3^2-\gamma _1^2\right) }{2\alpha _1^2}}{2\sqrt{\alpha _2^2-\left( \frac{\alpha _1^2+\alpha _2^2-\gamma _1^2}{2\alpha _1}\right) ^2}},\\ \delta _3&=\Bigg (\alpha _3^2-\left( \frac{\alpha _1^2+\alpha _3^2-\gamma _1^2}{2\alpha _1}\right) ^2- \frac{\left( \alpha _2^2+\alpha _3^2-\gamma _2^2-\frac{\left( \alpha _1^2+\alpha _2^2-\gamma _1^2\right) \left( \alpha _1^2+\alpha _3^2-\gamma _1^2\right) }{2\alpha _1^2}\right) ^2}{4\left( \alpha _2^2-\left( \frac{\alpha _1^2+\alpha _2^2-\gamma _1^2}{2\alpha _1}\right) ^2\right) }\Bigg )^{\frac{1}{2}}. \end{aligned}
We project the intersection of equation $$(E_1)$$ and $$(E_i)$$ to the hyperplane orthogonal to $$x_1$$ and suppose the projection region is $$D_{i-1}, i=2,3,4.$$ Then $$D=D_1\cap D_2\cap D_3,$$ where
\begin{aligned} D_1=&\{(x_2,x_3,\ldots ,x_n)\in \mathbb {R}^{n-1}|x_2^2+x_3^2+\cdots +x_n^2<1-a^2\}.\\ D_2^1=&\Big \{(x_2,x_3,\ldots ,x_n)\in \mathbb {R}^{n-1}|x_2^2+x_3^2+\cdots +x_n^2<1-\left( \frac{b-\beta _2x_2}{\beta _1}\right) ^2,x_2<\frac{b}{\beta _2}\Big \},\\ D_2^2=&\Big \{(x_2,x_3,\ldots ,x_n)\in \mathbb {R}^{n-1}|x_2^2+x_3^2+\cdots +x_n^2<1,x_2\ge \frac{b}{\beta _2}\Big \},\\ D_2=&D_2^1\cup D_2^2.\\ D_3^1=&\Big \{(x_2,x_3,\ldots ,x_n)\in \mathbb {R}^{n-1}|x_2^2+x_3^2+\cdots +x_n^2<1-\left( \frac{f-\delta _2x_2-\delta _2x_3}{\delta _1}\right) ^2,\\ ~~~~~~&~~~~~~f-\delta _2x_2-\delta _2x_3>0\Big \},\\ D_3^2=&\{(x_2,x_3,\ldots ,x_n)\in \mathbb {R}^{n-1}|x_2^2+x_3^2+\cdots +x_n^2<1,f-\delta _2x_2-\delta _2x_3\le 0\},\\ \quad D_3=&D_3^1\cup D_3^2. \end{aligned}

The region of $$(x_2,x_3)$$ for $$D$$ is the shadow of Fig. 4, and that of $$(x_4,\ldots ,x_n)$$ is an $$(n-3)$$-dimensional ball with radius $$r=\sqrt{1\!-a^2-\!\left( \frac{b-a\beta _1}{\beta _2}\right) ^2\!-\!\left( \frac{f-\delta _1a-\delta _2\frac{b-a\beta _1}{\beta _2}}{\delta _3}\right) ^2}.$$

For $$(x_4,\ldots ,x_n)$$, we adopt hyper sphere coordinate transformation. Let
$${\left\{ \begin{array}{ll} x_4=t\cos \varphi _1\\ x_5=t\sin \varphi _1\cos \varphi _2\\ ~~~~~\vdots \\ x_{n-1}=t\sin \varphi _1\cdots \sin \varphi _{n-5}\cos \varphi _{n-4}\\ x_{n}=t\sin \varphi _1\cdots \sin \varphi _{n-5}\sin \varphi _{n-4}\\ \end{array}\right. }$$
where $$0\le t \le r, 0\le \varphi _k\le \pi ,k=1,\ldots ,n-5,0\le \varphi _{n-4}\le 2\pi .$$
For a fixed $$t$$, denote by $$D(t)$$ the corresponding region of $$(x_2,x_3)$$ and by $$s(t)$$ the area of $$D(t)$$. Let $$f(t)$$ be the area of triangular $$\triangle _{P_1P_2P_3}$$, then $$s(t)\ge f(t)$$. Thus,
\begin{aligned} Q=&\int _0^r\int _0^{2\pi }\int _0^{\pi }\cdots \int _0^{\pi }t^{n-4}\sin \varphi _{n-5}\cdots \sin ^{n-4}\varphi _1 \int \!\!\!\!\int \limits _{D(t)} \frac{\text {d}x_2\text {d} x_3}{\sqrt{1-\sum _{i=2}^{n}x_i^2}}\text {d}\varphi _1\cdots \text {d}t\\ \ge&\int _0^r\int _0^{2\pi }\int _0^{\pi }\cdots \int _0^{\pi }t^{n-4}\sin \varphi _{n-5}\cdots \sin ^{n-4}\varphi _1 \int \!\!\!\!\int \limits _{D(t)}\text {d}x_2\text {d}x_3\text {d}\varphi _1\cdots \text {d}\varphi _{n-4}\text {d}t\\ =&2\pi \int _0^rt^{n-4}s(t)\text {d}t\prod _{k=1}^{n-5}\int _0^{\pi }\sin ^k\varphi \text {d}\varphi \ge 2\pi \int _0^rt^{n-4}f(t)\text {d}t\prod _{k=1}^{n-5}\int _0^{\pi }\sin ^k\varphi \text {d}\varphi , \end{aligned}
and
\begin{aligned} \begin{aligned}&\varGamma _n(\gamma _1,\gamma _2,\gamma _3)=\frac{Q}{|S^n|} \ge \frac{2\varGamma (\frac{n+2}{2})\prod _{k=1}^{n-5}\int _0^{\pi }\sin ^k\varphi \text {d}\varphi }{n\pi ^{n/2-1}}\int _0^rt^{n-4}f(t)dt. \end{aligned} \end{aligned}
(1)
We next give the lower bounds of $$\frac{2\varGamma (\frac{n+2}{2})\prod _{k=1}^{n-5}\int _0^{\pi }\sin ^k\varphi d\varphi }{n\pi ^{n/2-1}}$$ and $$\int _0^rt^{n-4}f(t)dt$$.
Since $$\int _0^{\pi }\sin ^k\varphi d\varphi =\sqrt{\pi }\frac{\varGamma ((k+1)/2)}{\varGamma (k/2+1)}$$ and $$\varGamma (x)$$ is increasing when $$x>2$$, we obtain
\begin{aligned} \frac{2\varGamma (\frac{n+2}{2})\prod _{k=1}^{n-5}\int _0^{\pi }\sin ^k\varphi d\varphi }{n\pi ^{n/2-1}} \ge \frac{2\varGamma (\frac{n+2}{2})}{\pi ^{3/2}n\varGamma (\frac{n-3}{2})}\ge \frac{n-3}{2\pi ^{3/2}}. \end{aligned}
(2)
For the lower bound of $$\int _0^rt^{n-4}f(t)dt$$, we first have the coordinate of $$P_1,P_2,P_3$$:
\begin{aligned} P_1&=\left( \frac{b-a\beta _1}{\beta _2},\sqrt{1-a^2-\left( \frac{b-a\beta _1}{\beta _2}\right) ^2-t^2}\right) \triangleq (a_1,b_1),\\ P_2&=\left( k_1(s_1-\sqrt{t_1-q_1t^2})+k_2,s_1-\sqrt{t_1-q_1t^2}\right) \triangleq (a_2,b_2),\\ P_3&=\left( \frac{f-a\delta _1-\delta _3(s_2-\sqrt{t_2-q_2t^2})}{\delta _2},s_2-\sqrt{t_2-q_2t^2}\right) \triangleq (a_3,b_3). \end{aligned}
where
\begin{aligned} h_1&=-\frac{\delta _3\beta _2}{\delta _1\beta _2-\delta _2\beta _1}\;,\quad h_2=\frac{f\beta _2-b\delta _2}{\delta _1\beta _2-\delta _2\beta _1},\quad k_1=\frac{\delta _3\beta _1}{\delta _1\beta _2-\delta _2\beta _1}\;,\quad k_2=\frac{b\delta _1-f\beta _1}{\delta _1\beta _2-\delta _2\beta _1},\\ s_1&=-\frac{h_1h_2+k_1k_2}{1+k_1^2+h_1^2}\;,\quad q_1=\frac{1}{1+k_1^2+h_1^2}, \quad t_1=\frac{1-h_2^2-k_2^2}{1+k_1^2+h_1^2}+\left( \frac{h_1h_2+k_1k_2}{1+k_1^2+h_1^2}\right) ^2,\\ s_2&=\frac{(f-\delta _1a)\delta _3}{\delta _2^2+\delta _3^2}\;,\quad q_2=\frac{\delta _2^2}{\delta _2^2+\delta _3^2}, \quad t_2=\left( 1-a^2-\frac{(f-\delta _1a)^2}{\delta _2^2+\delta _3^2}\right) \frac{\delta _2^2}{\delta _2^2+\delta _3^2}. \end{aligned}
The area of $$\triangle _{P_1P_2P_3}$$ is $$f(t)=\frac{1}{2}(a_1b_2+a_2b_3+a_3b_1-a_3b_2-a_2b_1-a_1b_3).$$ It can be verified that $$f(t)$$ is decreasing when $$t\in [0,r]$$ and $$f(r)=f'(r)=0, f''(r)>0$$. We have
\begin{aligned} \int _0^rt^{n-4}f(t)\text {d}t&\ge \int _0^{r-\frac{r}{n}}t^{n-4}f(t)\text {d}t \ge \frac{r^{n-3}}{n-3}(1-\frac{1}{n})^{n-3}f(r-\frac{r}{n}). \end{aligned}
Notice that $$\left( 1-\frac{1}{n}\right) ^{n-3}\ge \left( 1-\frac{1}{n}\right) ^{n}\approx e^{-1}$$ when $$n$$ is sufficiently large, and by Taylor series for $$f(r-\frac{r}{n})$$, $$f(r-\frac{r}{n})=\varTheta (\frac{1}{n^2}).$$ We have for some constant $$c''$$ unrelated to $$n$$,
\begin{aligned} \int _0^rt^{n-4}f(t)dt\ge \frac{c'' r^{n-3}}{n^2(n-3)}. \end{aligned}
(3)
Combining (1), (2) and (3), we have $$\varGamma _n(\gamma _1,\gamma _2,\gamma _3)\ge \frac{c''r^{n-3}}{2\pi ^{3/2}n^2}.$$ Now $$r$$ can be regarded as a function with respect to $$\alpha _1, \alpha _2, \alpha _3$$, where $$\gamma _3\le \alpha _1,\alpha _2,\alpha _3\le 1$$. It can be verified that $$r$$ decreases with $$\alpha _1,\alpha _2,\alpha _3$$ increasing. Let $$\alpha _1=1,\alpha _2=1,\alpha _3=1$$, we get the minimal value of $$r$$. $$r_{\min }=\sqrt{c_{\mathcal {H}_3}-\left( 1-\frac{\gamma _3^2}{2c_{\mathcal {H}_3}}\right) ^2}$$, $$c_{\mathcal {H}_3}=\gamma _2^2\left( 1-\frac{\gamma _2^2c_{\mathcal {H}_{1}}^2}{4}\right) .$$ So, $$\varGamma _n(\gamma _1,\gamma _2,\gamma _3)\ge \frac{c''r_{\min }^{n-3}}{2\pi ^{3/2}n^2}.$$

### Theorem 3

Let $$n$$ be a non-negative integer, $$N$$ be an integer and $$0.88<\gamma _3<1<\gamma _2<\gamma _1<\sqrt{2}\gamma _3$$, where $$\gamma _3$$ is very close to $$1$$. Let
$$N_{3}=c_3n^{3}(\frac{d_{\max }}{r_{\min }})^n,$$
where $$d_{\max }=\sqrt{1-\left( \frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) ^2- \left( \frac{1}{c_{\mathcal {H}_2}}\left( \frac{\gamma _3^2+1-\gamma _2^2}{2}- \frac{2\gamma _3^2-\gamma _1^2}{2\gamma _3}\frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) \right) ^2}, r_{\min }=\sqrt{c_{\mathcal {H}_3}-\left( 1-\frac{\gamma _3^2}{2c_{\mathcal {H}_3}}\right) ^2}, c_{\mathcal {H}_1}=\frac{1}{\gamma _1\sqrt{1-\frac{\gamma _1^2}{4}}}, c_{\mathcal {H}_2}=\frac{\gamma _{1}}{\gamma _3}\sqrt{1-\frac{\gamma _{1}^2}{4\gamma _3^2}}, c_{\mathcal {H}_3}=\gamma _2^2\left( 1-\frac{\gamma _2^2c_{\mathcal {H}_{1}}^2}{4}\right) ,$$ and $$c_3$$ is a positive constant unrelated to $$n$$. Let $$S$$ be a subset of $$C_n(\gamma _3 R)\cap B_n(\varvec{c}_{1},\gamma _{1} R)\cap$$ $$B_n(\varvec{c}_2,\gamma _2 R)\cap B_n(\varvec{c}_3,\gamma _3 R)$$ of cardinality $$N$$ whose points are picked independently at random with uniform distribution. If $$N_{3}<N<2^n$$, then for any subset $$C\subseteq S$$ of size at least $$N_{3}$$ whose points are picked independently at random with uniform distribution, with overwhelming probability, for all $$\varvec{v}\in S$$, there exists a $$\varvec{c}\in C$$ such that $$\Vert \varvec{v}-\varvec{c}\Vert \le \gamma _3 R$$.

### Proof

Combining Lemmas 2 and 3, we have
$$\varOmega _n(\gamma _1,\gamma _2,\gamma _3) =\frac{\varGamma _n(\gamma _1,\gamma _2,\gamma _3)}{\varGamma _n(\gamma _1,\gamma _2)} \ge \frac{c''}{\sqrt{\pi }n^2}\left( \frac{r_{\min }}{d_{\max }}\right) ^n.$$
Let $$N_{3}=c_3n^{3}(\frac{d_{\max }}{r_{\min }})^n$$, the remaining proof is similar to that of Theorem 2.
The Optimal Time Complexity. It can be proved that $$N_1N_2N_3(N_1+N_2+N_3)$$ decreases with $$\gamma _3$$. In fact,
• $$N_1=(\frac{1}{\gamma _1\sqrt{1-\gamma _1^2/4}})^n\lceil 3\sqrt{2\pi }n^{3/2}\rceil$$ is unrelated to $$\gamma _3$$.

• $$N_{2}=c_2(\frac{c_{\mathcal {H}_2}}{d_{\min }})^n\lceil n^{\frac{3}{2}}\rceil$$. Only $$c_{\mathcal {H}_2}=\frac{\gamma _{1}}{\gamma _3}\sqrt{1-\frac{\gamma _{1}^2}{4\gamma _3^2}}=\sqrt{1-(1-\frac{\gamma _1^2}{2\gamma _3^2})^2}$$ is related to $$\gamma _3$$, and it is easy to see that $$c_{\mathcal {H}_2}$$ decreases with respect to $$\gamma _3$$, which implies that $$N_2$$ is a monotonically decreasing function of $$\gamma _3$$.

• $$N_{3}=c_3n^{3}\left( \frac{\sqrt{1-\left( \frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) ^2- \left( \frac{1}{c_{\mathcal {H}_2}}\left( \frac{\gamma _3^2+1-\gamma _2^2}{2}- \frac{2\gamma _3^2-\gamma _1^2}{2\gamma _3}\frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) \right) ^2}}{\sqrt{c_{\mathcal {H}_3}-\left( 1-\frac{\gamma _3^2}{2c_{\mathcal {H}_3}}\right) ^2}}\right) ^n$$. First, the denominator of $$N_3$$ increases with $$\gamma _3$$, since $$c_{\mathcal {H}_3}$$ is unrelated to $$\gamma _3$$. By $$\gamma _1>1$$, we have $$\left( \frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) ^{'}=\frac{\gamma _3^2+\gamma _1^2-1}{2\gamma _3^2}>0$$, and $$\left( \frac{\gamma _3^2+1-\gamma _2^2}{2}- \frac{2\gamma _3^2-\gamma _1^2}{2\gamma _3}\frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}\right) ^{'} =\gamma _3-\frac{2\gamma _3^2+\gamma _1^2}{2\gamma _3^2}\frac{\gamma _3^2-\gamma _1^2+1}{2\gamma _3}-\frac{2\gamma _3^2-\gamma _1^2}{2\gamma _3}\frac{\gamma _3^2+\gamma _1^2-1}{2\gamma _3^2} =\frac{\gamma _1^2(\gamma _1^2-1)}{2\gamma _3^3}>0$$. Together with $$\frac{1}{c_{\mathcal {H}_2}}$$ increases with $$\gamma _3$$, then we have the numerator of $$N_3$$ decreases with $$\gamma _3$$. Thus, $$N_3$$ decreases with respect to $$\gamma _3$$.

Therefore, $$N_1N_2N_3(N_1+N_2+N_3)$$ decreases with $$\gamma _3$$.

Since the expression of the time complexity is complicated, we solve a numerical optimal solution. Take $$\gamma _3=1$$. Let $$\gamma _1$$ go through from 1 to 1.414 by 0.0001 and for a fixed $$\gamma _1$$, let $$\gamma _2$$ go through from 1 to $$\gamma _1$$ by 0.0001, then we can easily find the minimal value of the exponential constant for the running time. Thus, we obtain the numerical optimal time complexity of our three-level sieve algorithm.

### Theorem 4

The optimal time complexity of the algorithm is $$2^{0.3778n+o(n)}$$ poly-nomial-time operations with $$\gamma _3\rightarrow 1,\gamma _1=1.1399,\gamma _2=1.0677$$, and the corresponding space complexity is $$2^{0.2833n+o(n)}$$ polynomially many bits under Heuristic Assumption 1.

### Remark 1

As in [20], the number of iterations is usually linear in the dimension of lattices. Regardless of the number of iterations, the polynomial factors hidden in the time complexity in NV algorithm and WLTB algorithm are respectively $$n^{3}$$ and $$n^{4.5}$$. In our three level sieve algorithm, the polynomial parts of $$N_1, N_2$$ and $$N_3$$ given by Theorem 1, 2, and 3 are $$n^{3/2}, n^{3/2}$$ and $$n^{3}$$ respectively. So the hidden polynomial factor in our algorithm is $$n^{9}$$ without the number of iterations.

### Remark 2

It is natural to extend the three-level sieve algorithm to multiple-level, such as four-level algorithm. However, the number of small balls will increase as the number of the levels increases. Therefore, we conjecture that the time complexity may be decreased with small number levels, but will increase if the number of levels is greater than some positive integer.

## 4 Experimental Results

### 4.1 Comparison with the Other Heuristic Sieve Algorithms

We implemented the NV algorithm, the WLTB algorithm and our three-level sieve algorithm on a PC with Windows 7 system, 3.00 GHz Intel 4 processor and 2 GByte RAM using Shoup’s NTL library version 5.4.1 [28]. Instead of implementing the GaussSieve algorithm, we directly applied the GaussSieve Alpha V.01 published by Voulgaris [29] on a PC with Fedora 15 system, 3.00 GHz Intel 4 processor and 2 GByte RAM.

We performed experiments to compare our three-level sieve algorithm with the other three algorithms. For every dimension $$n$$, we first used the method in [18] to pick some random $$n$$-dimensional lattice and computed the LLL-reduced basis, then we sampled the same number of lattice vectors, and performed the NV algorithm with $$\gamma =0.97$$, the WLTB algorithm with $$\gamma _1=1.0927,\gamma _2=0.97$$ and our three-level sieve algorithm with $$\gamma _1=1.1399,\gamma _2=1.0667,\gamma _3=0.97$$ using these samples. We performed one experiments on lattices with dimension 10, 20 with more than 100000 samples, but about fifty experiments with fewer samples, and two experiments on dimension 25, 30, 40, 50. Instead of using our samples, we just performed the GaussSieve Alpha V.01 with the selected lattices as its inputs. The experimental results of the four algorithms are shown in Table 2, where $$\varvec{v}$$ is the output vector of the corresponding algorithm.

In our experiments, the GaussSieve algorithm is much faster than the others and succeeds to find the shortest vectors for all the lattices we picked. Besides of the major reason that the GaussSieve algorithm performs better in practice (it has been reported that the GaussSieve algorithm is more efficient than the NV algorithm), another possible reason is that our implementation is a little poor.
Table 2.

Experimental results.

Dimension

10

20

25

30

40

50

60

Number of sample

150000

100000

8000

5000

5000

3000

2000

Time of sample (s)

301

810

87833

73375

147445

120607

167916

Time (s)

NV algorithm

25005

64351

120

220

625

254

187

WLTB algorithm

23760

18034

35

42

93

46

47

Our algorithm

20942

13947

27

27

57

29

30

GaussSieve algorithm

0.003

0.013

0.068

0.098

0.421

3.181

42.696

$$\frac{\Vert \varvec{v}\Vert }{\lambda _1}$$

NV algorithm

1

1

23.8

38.3

170.1

323

347.7

WLTB algorithm

1

1

25.9

35.1

170.1

323

347.7

Our three-level algorithm

1

1

21.2

38.3

170.1

323

347.7

GaussSieve algorithm

1

1

1

1

1

1

1

Compared with the NV and WLTB algorithms, it seems that our algorithm may be slower for low dimensional lattices due to the larger hidden polynomial factor. However, on one hand, the number of sieved vectors in each iteration of our algorithm decreases faster because the number of small balls is larger, which implies that the number of iterations is smaller and the number of the vectors to be sieved in the next iteration is smaller as well. On the other hand, the time complexity is for the worst case. In practice, we need not to check all the big balls, medium balls and small balls to decide which small ball the sieved vector belongs to. Thus, with the same number of samples in our experiments, our algorithm runs faster than the NV and WLTB algorithms. Since the sample procedure is very fast when the dimension $$n$$ is not greater than twenty, we can sample enough lattice vectors to ensure that the three algorithms can find a shortest nonzero lattice vector. In such case, the time of sieving overwhelms the time of sampling, so our algorithm usually costs the least total time.

### 4.2 On Heuristic Assumption 1

To test the validity of the Heuristic Assumption 1 that the distribution of the sieved vectors remains uniform, we picked four random lattices of dimension 10, 25, 40 and 50, sampled 150000, 8000, 5000, 3000 lattice vectors and then sieved them respectively. As in [20], we plotted the number of sieved vectors in each iteration (see Fig. 5). It can be seen that the head and the tail of the curve change slightly, but most of the curve, the middle part, decreases regularly. The lost vectors in each iteration are those used as centers or reduced to zero which means collisions occur. So the curve shows that the numbers of centers and collisions in most of the iterations are nearly the same, which partially suggests that the distribution of the sieved vectors is close to uniform throughout the iterations.

## 5 Conclusion

In this paper, we propose a three-level heuristic sieve algorithm to solve SVP and prove that the optimal running time is $$2^{0.3778n+o(n)}$$ polynomial-time operations and the space requirement is $$2^{0.2833n+o(n)}$$ polynomially many bits under Heuristic Assumption 1.

## Notes

### Acknowledgement

We like to thank Michael Schneider very much for his valuable suggestions on how to improve this paper. We also thank the anonymous referees for their helpful comments. We are grateful to Panagiotis Voulgaris for the publication of his implementation of the GaussSieve algorithm. Pan would like to thank Hai Long for his help on the programming.

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