Wavelet Based Solutions to the Poisson and the Helmholtz Equations on the n-Dimensional Unit Sphere

We present a method of solving partial differential equations on the n-dimensional unit sphere using methods based on the continuous wavelet transform derived from approximate identities. We give an explicit analytical solution to the Poisson equation and to the Helmholtz equations. For the first one and for some special values of the parameter in the latter one, we derive a closed formula for the generalized Green function.


Introduction
In the last thirty years, many authors developed wavelet methods for solving differential equations. Already in the 1990 s numerical solutions of ODEs [10,34] and PDEs [27,32] on the Euclidean space were found. Some further examples of wavelet application to ODEs are presented in [20,28], and to PDEs in [5,16,19,21,30,39,42]. Unlike in the articles listed so far, the papers [9,17,29] 1 of PDEs on the sphere. Recent years have brought a number of papers in which wavelet methods were involved to solving fractional differential equations [1,18,33,41]. The list is far from being complete, but it is apparent that wavelet based methods for solving PDEs are usually numerical. No publication is known to us that presents an analytical solution.
Theories of continuous spherical wavelets have been developed in the last decades, simultaneously to theories of wavelets over the Euclidean space. Iglewska-Nowak has shown in [23,Section 5] that there exist only two essentially different continuous wavelet transforms for spherical signals, namely that based on group theory [2,3] and that derived from approximate identities [7,11,[22][23][24]. In the present paper we show that the latter one can be efficiently applied to solving partial differential equations on the sphere. We give an explicit solution to the Poisson and the Helmholtz equations on the unit sphere of arbitrary dimension in form of a convolution with the Green function. The Green function is given as a series but in the case of the Poisson equation and for some values of the parameter a in the Helmholtz equation its closed form is derived.
The Helmholtz equation * u + au = f , with * being the Laplacian, naturally appears from general conservation laws of physics and can be interpreted as a wave equation for monochromatic waves (wave equation in the frequency domain). Applying standard separation of variables, one can reduce some time-dependent equations, including heat conduction equation, Schrödinger equation, telegraph and other wavetype, or evolutionary, equations, to a Helmholtz equation with a ∈ C. Here we study the case a = k 2 for some k 2 ∈ R, obtaining a closed form of the Green function for the Helmholtz equation, including some negative values of the parameter a, i.e., a purely imaginary k. As far as we know, the closed form for the Green function for negative a has not been given in literature before. This specific case is important for its applications, for instance, it appears in the telegraph equation and corresponds to two types of waves: exponentially growing and exponentially decaying. The relativistic counterpart of the Schrödinger equation, called the Klein-Gordan equation, describing a free particle with zero spin in the frequency domain, also leads to the Helmholtz equation with purely imaginary k.
The present paper seems to be the first attempt to involve wavelet methods to analytical solving of PDEs. The results concerning the Poisson equation are well known for the 2-dimensional sphere but to our best knowledge a generalization to the case of Helmholtz equation and to higher dimensions has not been published so far.
The paper is organized as follows. Section 2 contains basic information about functions and differential operators on the sphere. In Sect. 3 we introduce the continuous spherical wavelet transform based on approximate identities. Main results yielding a solution to the Helmholtz and the Poisson equations, contained in Theorem 1 and Theorem 2, are presented in Sect. 4 together with a list of closed representations of the Green function for various dimensions and various values of parameter a. The Appendix 1 contains some integral formulae that can be useful for derivation of closed form of the Green function.

Preliminaries
A square integrable function f over the n-dimensional unit sphere S n ⊆ R n+1 , n ≥ 2, with the rotation-invariant measure dσ normalized such that can be represented as a Fourier series in terms of the hyperspherical harmonics, Fourier coefficients of f . The hyperspherical harmonics of degree l and order k are given by for some constants A k l . Here, (ϑ 1 , . . . , ϑ n−1 , ϕ) are the hyperspherical coordinates of x ∈ S n , x n = sin ϑ 1 . . . sin ϑ n−2 sin ϑ n−1 cos ϕ, x n+1 = sin ϑ 1 . . . sin ϑ n−2 sin ϑ n−1 sin ϕ, and C K κ are the Gegenbauer polynomials of degree κ and order K . Zonal (rotation-invariant) functions are those depending only on the first hyperspherical coordinate ϑ = ϑ 1 . Unless it leads to misunderstandings, we identify them with functions of ϑ or t = cos ϑ. A zonal L 1 -function f has the following Gegenbauer expansion where f (l) are the Gegenbauer coefficients of f and λ is related to the space dimension by Journal of Fourier Analysis and Applications (2023) 29 :28 Consequently, for a zonal L 2 -function f one has compare (1), (2), and (3). For f , g ∈ L 1 (S n ), g zonal, their convolution f * g is defined by and for f ∈ L 2 (S n ) it is equal to If f is a zonal function, then The Laplace-Beltrami operator * on the sphere is defined by * f (ϑ 1 , . . . , ϑ n−1 , ϕ) It is known that the hyperspherical harmonics are the eigenfunctions of * , i.e., * Y k see [35, Chapter II, Theorem 4.1]. The relation of * and the Laplace operator is given by where R ≥ 0 is the radial distance of x ∈ R n in the hyperspherical coordinates, see [35, Chapter II, Proposition 3.3]. The Laplace operator is commutative with SO(n + 1)-rotations ϒ, Since S n is a manifold without boundary, the Green second surface identity implies that for f , g of class C 2 the following holds: The scalar product in L 2 (S n ) is antilinear in the first variable, Since * is a linear operator, one has * f = * f and (9) can be also written as

The Wavelet Transform
The wavelet transform based on approximate identities has been developed starting from the 1990 s by Freeden et al. [11,14,15] and Bernstein et al. [6,7], as well as by Iglewska-Nowak in the recent years [22,23]. Its modified version can be found in [24,25], where two distinct wavelet families are used to analysis and synthesis of a signal. In the present paper we shall use the definition from [24] adapted to the case of rotation-invariant functions with 0-mean and weight function α(ρ) = 1 ρ and simplified as in [22] (the convergence proof of [22,Theorem 3.2] can be easily adapted to the considered case).

Definition 1 Families
Definition 2 Let { ρ } ρ∈R + and { ρ } ρ∈R + be an admissible wavelet pair. Then, the spherical wavelet transform is defined by The wavelet transform is invertible (in L 2 -sense) by If = , this definition reduces to the classical one [11,23].
Then, it is a wavelet family with the reconstruction wavelet { ρ } ρ∈R + given by Example 1 An example of wavelets being their own reconstruction family are the Poisson wavelets of order d ∈ N [26], They are given explicitly as derivatives of the Poisson kernel by

The Poisson and the Helmholtz Equation
Our aim in this section is to study the Helmholtz equation * u + au = f for a ∈ R and the Poisson equation * u = f . We start with applying wavelet methods and in the second part of the section we give explicit solutions for these equations in form of convolutions with the Green functions.

Solution with Wavelet Methods
Using the wavelet transform and the inverse wavelet transform with suitably chosen wavelets, we may derive a solution to the Helmholtz and the Poisson equation. where (2) Suppose that f ∈ C(S n ), u ∈ C 2 (S n ) are such that f = u = 0 and satisfy * u = f . Then, Remark 2 1. The obtained result coincides with the one presented in [4, Chap. IV, Par. 5], concerning the general case of 'suitable smooth' functions in a vector space. 2. If a = L(n + L − 1) for some L ∈ N 0 , then (14) can be solved only if The solution is unique if one additionally assumes u * C L = 0 and it is given by (17) with compare [13,Section 4.1] or [12,Section 4.6] for the Poisson equation on the two-dimensional sphere (note that the factor 1 4π is included in the convolution definition (4)), resp. [37,38] The wavelet transform of f with respect to ρ is equal to and it can be written as (by the commutativity of the Laplace-Beltrami operator with rotations (8)). By (6) and (2), the Gegenbauer coefficients of the family ρ := ( * +ā) ρ are equal to Since a = (n−1)l+l 2 for all l ∈ N, the family { ρ } is a wavelet with the reconstruction wavelet { ρ } given by compare (12). It follows from (21) that and hence, in the case u = 0, 1/R R . L 2 -convergence of the triple integral is ensured by the properties of the wavelet families, thus, the order of integration can be changed and one obtains Now, by (5), Since ρ * ρ ∈ L 2 (S n ) ⊂ L 1 (S n ), it can be integrated term by term in its Gegenbauer representation, i.e., If u = 0 and a = 0, it follows directly from (14) that u(0) = 1 a · f (0), i.e., u given by (17) with G satisfying (18) for all l ∈ N 0 solves the Helmholtz equation.

Closed Forms of the Green Functions
In the papers [37,38] closed forms of the Green functions (19) for the Helmholtz equation with a = L(n + L − 1) for some L ∈ N 0 are given. In the remaining part of the paper we develop a method for finding closed expressions of these functions. It is based on the series representation of the Poisson kernel and has been used for finding explicit formulae of various wavelets. Contrary to the one from [37,38], it is applicable also in the case a = L(n + L −1). For the set of indices that we have tested, the expressions for G coincide with those derived by Szmytkowski. For the Poisson equation on the 2-sphere we obtain the same Green function as the authors of [12,13,31]. Then Proof According to (13), the integrand Further, for (i.e., l > L 0 ) we have Condition (25) ensures convergence of the integral on the left-hand-side of (26) and well definiteness of the quotient Since the Gegenbauer polynomials C λ l over the interval [−1, 1] are bounded by |C λ l (cos ϑ)| ≤ (n + l − 2) n−2 (27) uniformly in ϑ ∈ [0, π] (compare [36, Theorem 7.33.1]), the series (24) is absolutely convergent for r ∈ [0, 1). Consequently, the first summand in (23) equals and (23) coincides with (22).

Corollary 3
Let n ∈ N, n ≥ 2, be fixed, λ = n−1 2 , and suppose that a = L(n + L − 1) for some L ∈ N 0 . Further, let G denote the function
For nonzero a, the numbers L and L are of opposite sign and one can choose L to be positive in order to apply Corollary 3. If a = 0, take L = 0. Consequently, For t = cos ϑ it can be expressed as This coincides (up to the constant 1 4π ) with the result from [13,Lemma 4.3] or [12, Lemma 4.6.2]. Table 2 gives the expressions for G(cos ϑ) for several values of parameters n and positive integer L. They coincide with those derived in [37,38].
In Table 3 closed forms of the Green function for the Helmholtz equation are given for some positive rational (noninteger) values of the parameter a. Table 4 collects closed forms of the Green function for the Helmholtz equation for some negative values of the parameter a.
Theorem 2 and Corollary 3 hold for all dimensions and all a's, but a closed expression on the right-hand-side of (23) can be rarely obtained for noninteger a. Nevertheless, in this way we can compute the Green function of the Helmholtz equation for a quite wide range of indices.
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Appendix 1: Some Helpful Integral Formulae
In this section, we collect several hints for computing integrals of irrational functions that arise when one wants to derive a formula for the kernel G in an even-dimensional space. Table 2 The Green function G for different values of n and positive integer L; t = cos ϑ n = 2,  Table 2 continued n = 6, Table 3 The Green function G for different values of n and positive rational a; t Table 4 The Green function G for different values of n and negative a; t = cos ϑ Lemma 4 If λ is a half-integer, λ ∈ N/2 \ N, then the following holds: where r = r − t and t = 1 − t 2 , and polynomials Q λ j , j = 0, 1, . . . , λ − 3 2 , are given recursively by Lemma 5 Suppose t = 0 and consider the integral dR.