Abstract
In this paper we consider extensions of positive operators. We study the connections between the von Neumann theory of extensions and characterisations of positive extensions via decompositions of the domain of the associated form. We apply the results to elliptic second order differential operators and look in particular at examples of the Laplacian on a disc and the Aharonov–Bohm operator.
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1 Introduction
Let A be a closed strictly positive symmetric operator with dense domain D(A) and range R(A) in a Hilbert space H. In [11, 12], Krein proved that there is a one to one correspondence between the set of positive self-adjoint extensions \(A_B\) of A and a set of pairs \(\{N_B, B\}\), where \(N_B\) is a subspace of the kernel N of \(A^{*}\) and B is a positive self-adjoint operator with domain and range in \(N_B\). Krein’s result was subsequently developed further by Visik [15] and Birman [3]; this work of the three authors will be referred to as the KVB theory. An important extension of the KVB theory was made in [8] to a pair of closed densely defined operators \(A, A'\), which form a dual pair in the sense that \(A \subset (A')^{*}\) and are such that \(A \subset A_{\beta } \subset (A')^{*}\) for an operator \(A_{\beta }\) with a bounded inverse. The results in [8] include those of KVB when \(A=A'\). Of particular interest to us in [8] is the application of the abstract theory to the case when A is generated by an elliptic differential expression acting in a bounded smooth domain \(\Omega \) in \(\mathbb {R}^n\). In this case the self-adjoint extensions of A are determined by boundary conditions on the boundary \(\partial \Omega \) of \(\Omega \).
In [5], results in Rellich [13], Kalf [9] and Rosenberger [14] were applied to the KVB theory to determine all the positive self-adjoint extensions of a positive Sturm–Liouville operator with minimal conditions on the coefficients. Our objective in this paper is to investigate what can be achieved by applying similar methods to two problems on bounded domains in \(\mathbb {R}^n, n \ge 2\); in the first A is generated by a second order elliptic expression, and in the second it is the Aharonov–Bohm operator on a punctured disc. Our analysis depends on an abstract result which incorporates the von Neumann theory concerning all the self-adjoint extensions of any symmetric operator.
Denote by \(A_F,~ a_F[\cdot ,\cdot ]\) the Friedrichs extension and associated sesquilinear form of A. Then for all \(u \in D(A_F)\) and \(v \in Q(A_F)\) we have
where \((\cdot ,\cdot )\) is the inner product of H, and \(D(A_F)\) is dense as a subspace of \(Q(A_F)\) with inner product \(a_F[\cdot ,\cdot ]\) (see [7, Chapter IV] for more on the relation between sesquilinear forms, operators and their Friedrichs extension). By the KVB theory, \(\hat{A}\) is a positive self-adjoint extension of A if and only if, \(\hat{A} = A_B\), where B is a positive self-adjoint operator acting in a subspace \( N_B\) of N and \(A_B,~B\) have associated forms \(a_B,~b\), respectively which satisfy
Thus any \(u \in Q(A_B)\) can be uniquely written as \( u=u_F + u_N\), where \( u_F \in Q(A_F),~ u_N \in Q(B) \). There are two distinguished positive self-adjoint extensions of A, namely the Friedrichs (or strong) extension \(A_F\) and the Krein–von Neumann (or weak) extension \(A_K\). These are extremal in the sense that any positive self-adjoint extension \(\hat{A}\) of A satisfies \(A_K \le \hat{A} \le A_F\) in the form sense. In (1.1), the Krein–von Neumann extension \(A_K\) corresponds to \(B=0,~ N_B = N\), and the Friedrichs extension \(A_F\) to \(B= \infty ,~Q(B) = 0,\) that is, B acts trivially on a zero dimensional space.
2 Positive Extensions and the Von Neumann Theory
The von Neumann theory characterises the self-adjoint extensions of any closed densely defined symmetric operator T. Denoting the deficiency spaces \( \text {ker}(T^{*}\mp iI)\) by \(N_{\pm }\), we have
and \(T_S\) is a self-adjoint extension of T if and only if there is a unitary operator \(U(T_S){:}\,N_+ \rightarrow N_-\) such that
Let \(u, v \in D(T^{*})\). Then by the von Neumann theory, there exist unique \(u_0,v_0 \in D(T)\) and \(u_\pm ,v_\pm \in N_\pm \) such that \(u= u_0+u_+ + u_-\) and \(v= v_0+v_+ + v_-\).
It follows that
Let \(P_+\) and \(P_-\) denote the projections from \(D(T^{*})\) to \(N_+\) and \(N_-\) with respect to the decomposition (2.1) and let \(U{:}\,N_+\rightarrow N_-\) be unitary. Set \(\tilde{\Lambda }_0 =UP_++P_-\) and \(\tilde{\Lambda }_1=-iUP_++iP_-\). Then, for any \(u,v\in D(T^{*})\)
(see [10, Theorem 3]). The triple \((N_+, \tilde{\Lambda }_0,\tilde{\Lambda }_1)\) is a boundary triple (also known as a space of boundary values) for T.
Given a self-adjoint extension \(T_S\) of T, we now choose
Then, from (2.2), \(\ker \Lambda _1(T_S) = \mathcal D(T_S)\) and we obtain, for all \(u,v \in D(T^{*})\)
Let \(T=A\) be positive and B a positive self-adjoint operator on a subspace \(N_B\) of the kernel of \(A^{*}\) with domain D(B). By [2, Theorem 3.1], the domain of the self-adjoint extension \(A_B\) of A corresponding to B is
Remark 2.1
The special case \(B=0, N_B =N\) gives the domain of the Krein–von Neumann extension \(A_K\), namely
the sum being a direct sum since A is strictly positive. It follows that
The Friedrichs extension is characterised by the choice of B as acting trivially on \(N_B=\{0\}\). Following the approach of [2], we can set \( b[u] = \infty \) for \(u \in N{\setminus } Q(B)\). It follows from (1.1) that \(Q(A_B) =Q(A_F )\) if and only if \(Q(B) = \{0\}\). Since \(A_F\) is the only self-adjoint extension of A with domain in \(Q(A_F)\) it follows that its domain is determined by \(b[u] = \infty \) for all \(u \in N {\setminus }\{0\}\).
Theorem 2.2
Let \(A_B\) be a positive self-adjoint extension of the positive operator A associated with the pair \(\{B,N_B\}\). Let \(u \in D(A_B)\), where \(u=u_F + w\), \(u_F = u_0 +A_F^{-1}(Bw+v), u_0 \in \mathcal {D}(A), w \in \mathcal {D}(B), v \in N \cap \mathcal {D}(B)^{\bot }\). Then
where \(\Lambda _0(A_B) = U(A_B)P_+ +iP_-\) and \(\Lambda _1(A_B) =-iU(A_B)P_+ +i P_-\).
Proof
Let \(\varphi =\theta +\zeta \in Q(A_B)\) with \(\theta \in Q(A_F)\) and \(\zeta \in Q(B)\). Then on the one hand, we have
since \( w\in N\), and on the other hand,
On combining (2.11) and (2.12) we get
and as \(A^{*}\zeta =0\), Eq. (2.6) yields
Since \(\ker \Lambda _1(A_B) = D(A_B)\), (2.10) follows. \(\square \)
Let \(\{\psi _k\}\) be an orthonormal-basis of Q(B), where B is a positive self-adjoint operator in \(N_B\subset N\), and let \(w =\sum _j w_j \psi _j,~ \zeta =\sum \zeta _k\psi _k\) and \(b_{jk} =b[\psi _j,\psi _k]\). Then \(b[w,\zeta ] = \sum _{j,k} b_{jk}w_j \overline{\zeta _k}\) and from (2.10) and the fact that \(\ker \Lambda _1(A_B) = D(A_B)\), \( u = u_F + w \in D(A_B)\) if and only if
3 Elliptic Differential Operators of Second Order
In this section we shall apply the above abstract theory to the case when A is the closure of a symmetric second-order differential operator in \(L^2(\Omega )\) defined by
subject to conditions on the coefficients \(p_{ij},~q\) and the domain \(\Omega \). The assumptions are the ones made in [1] which weaken the smoothness requirements on the coefficients and the boundary of \( \Omega \) made by Grubb [8]. In the following definition of a boundary regularity class, \(B^{s}_{p,q}\) is the Besov space of order s (see [1, Section 2]), and we set \(x=(x',x_n), x'\in \mathbb {R}^{n-1}, x_n \in \mathbb {R}\).
Definition 3.1
The boundary \(\partial \Omega \) is said to be of class \(B^{M-\frac{1}{2}}_{p,q}\) if for each \(x\in \partial \Omega \) there exist an open neighbourhood U satisfying the following: for a suitable choice of coordinates on \(\mathbb {R}^n\), there is a function \(\gamma \in B^{M-\frac{1}{2}}_{p,q}(\mathbb {R}^{n-1})\) such that \(U\cap \Omega = U\cap \mathbb {R}^n_\gamma \) and \(U\cap \partial \Omega = U\cap \partial \mathbb {R}^n_{\gamma } \), where \( \mathbb {R}^{n}_{\gamma } = \{x \in \mathbb {R}^n{:}\,x_n > \gamma (x')\}\).
In the list of assumptions to be made, we shall denote the boundary of \(\Omega \) by \(\Sigma \), and \( H^s_t\) is a Bessel potential space (a Sobolev space for \(s \in \mathbb {N}\)), which we write as \(H^s\) when \(t=2\); see [1, Section 2] for definitions of \(H^s_t(\Omega ) \) and \(H^s_t(\Sigma )\).
Assumptions
-
1.
There exists \(c_0>0\) such that for all \(x\in \Omega \) and \(\xi \in \mathbb {R}^{n}\)
$$\begin{aligned} \sum _{i,j=1}^n p_{ij}(x)\xi _i\xi _j dx \ge c_0 \Vert \xi \Vert ^2. \end{aligned}$$ -
2.
There exists \(c >0\) such that
$$\begin{aligned} \Vert u\Vert ^2_1 = \int _{\Omega } \left( p |\nabla u|^2 + q|u|^2 \right) dx \ge c \Vert u\Vert ^2,\ \ \ u \in C_0^{\infty }(\Omega ). \end{aligned}$$The completion of \(C_0^{\infty }(\Omega )\) with respect to the norm \( \Vert \cdot \Vert _1 \) is the form domain \(Q(A_F)\) of A.
-
3.
The boundary \(\Sigma \) is of class \(B^{\frac{3}{2}}_{r,2}\) and the coefficients p and q of A lie in \(H^1_t(\Omega )\) and \(L_t(\Omega )\), respectively, under the constraints \(n\ge 2\), \(2< r<\infty \), \(2< t \le \infty \), and
$$\begin{aligned} 1-\tfrac{n}{t}\ge \tfrac{1}{2}-\tfrac{n-1}{r}> 0. \end{aligned}$$(3.2)
Remark 3.2
Our third assumption is Assumption 2.18 in [1]. Therefore, we have that for \(v \in Q(A_F)\), \(\gamma _0 v =0\), where \(\gamma _0\) is the trace operator which maps v into its value on \(\Sigma \) (see [1, Theorem 2.11]). Moreover, in the notation of [1, 6], denote the solution of
by
Then by [1, Theorem 5.4], for all \( s \in [0,2]\),
is continuous,
is a homeomorphism, and
We remark that under the more restrictive assumptions that \(\Omega \) is a bounded domain whose boundary is an \((n-1)\)-dimensional \(C^{\infty }\) manifold, and the coefficients \(p_{jk},~q\) of \(A'\) in (3.1) lie in \(C^{\infty }(\overline{\Omega })\) these properties were already shown by Grubb in [8].
Theorem 3.3
Let the above assumptions hold and let \(A_B\) be a positive self-adjoint extension of A. For \(u \in D(A_B)\), we have \(u=u_F+w\) for some \(u_F \in D(A_F),~w \in Q(B)\), and for all \(\zeta \in Q(B)\)
If \(\{\psi _k\}\) is an orthonormal basis of Q(B) then, with \(b_{jk}\) as in (2.14),
Proof
Let \(a_B[\cdot ,\cdot ], a_F[\cdot ,\cdot ], b[\cdot ,\cdot ]\) denote the forms associated with \( A_B, A_F, B\), respectively. For \( u, \varphi \in Q(A_B) \) we have the decompositions
If \( u \in D(A_B)\), it has the decomposition \(u=A_F^{-1}A^{*}u+(u-A_F^{-1}A^{*}u)\), i.e., \(u_F=A_F^{-1}A^{*}u\) and \(w=u-A_F^{-1}A^{*}u,\) since \(u_F\in D(A^{*})\cap Q(A_F) = D(A_F)\) and \(w\in Q(A_B)\cap N=Q(B)\). Now, let \(\varphi =\varphi _F+\zeta \in Q(A_B)\). Then
and furthermore,
Combining (3.11) and (3.12), we get
Now let \(\{\psi _k\}\) be an orthonormal-basis of Q(B), \(w=\sum w_j\psi _j\) and \(\zeta =\sum \zeta _k\psi _k\). Then
while the right-hand side of (3.13) is
Consequently
and equivalently,
As \(\{\zeta _k\}\) is an arbitrary sequence in \( \ell ^2\), the ‘boundary condition’ associated with \(A_B\) is given by
\(\square \)
Corollary 3.4
The boundary condition associated with \(A_B\) is given by
where \(\nu _1 u = \sum _{j,k =1}^n n_j \gamma _0[p_{jk}D_k u]\) and \(\mathbf{n} = (n_1,\ldots ,n_n)\) is the interior unit normal.
Proof
Since \(u,\psi _k\in D(A^{*})\), we can use [1, Theorem 6.1] to write
where \( \Gamma _1\) is the “regularised” Neumann operator given by \(\Gamma _1 u = \nu _1 u - P_{\gamma _0, \nu _1} \gamma _0 u\) and \(P_{\gamma _0, \nu _1} \gamma _0 \) is the Dirichlet-to-Neumann map \(P_{\gamma _0, \nu _1} \gamma _0 = \nu _1 K_{\gamma }^0\). Under the smoothness conditions assumed, the trace maps \(\gamma _0,~ \Gamma _1\), map \( D(A^{*})\) continuously into \( H^{-1/2}(\Sigma ),~ H^{1/2}(\Sigma )\), respectively. The terms on the right-hand side of (3.16) therefore represent in fact, \(H^{-1/2},H^{1/2}\)-duality products over the boundary \(\Sigma \), which are extensions of the \(L^2(\Sigma )\) inner products (see [1, Theorem 6.1]).
Since \((\nabla p \nabla -q )\psi _k=0\) and \(u_F\in Q(A_F)\), two of the four terms in (3.16) vanish and, as \(\Gamma _1 u_F=\nu _1 u_F\), we get that the boundary condition in (3.14) becomes
\(\square \)
Remark 3.5
The Friedrichs extension is determined by the boundary condition \(\gamma _0 u=0\). Under the additional smoothness assumptions on \(\Omega \) and the coefficients of \(A'\) in (3.1) in [8], the Friedrichs extension has domain \(H_0^1(\Omega ) \cap H^2(\Omega )\).
Remark 3.6
The Krein–von Neumann extension corresponds to \(B=0, Q_B = N_B = N =\text {ker}(A^{*})\) and so
when \( u=u_F+w, u_F \in Q(A_F), w \in N\). Thus in (3.15), \(b_{jk} =0\) for all j, k and \(\nu _1 u_F = \Gamma _1 u_F \). Since \(\nu _1 \) maps \(D(A^{*})\) continuously into \(H^{-1/2}(\Sigma )\) and \(\gamma _0\) is a homeomorphism of N onto \(H^{1/2}(\Sigma )\), it follows from (3.15) that the boundary condition satisfied by the Krein–von Neumann extension is
Since \( w = K_{\gamma }^0 \gamma _0 u\) we have
Remark 3.7
On combining (2.10) and (3.17) we have
For the Krein–von Neumann extension \(\Lambda _K\psi _k=0\), so we again get
as the Krein–von Neumann boundary condition.
Example 3.8
We consider extensions of the positive operator \(A=-\Delta +1\) when \(\Omega \) is the unit disc in \(\mathbb {R}^2\). According to (3.15), \(v=v_F+w\) lies in the domain of an extension \(A_B\) if and only if
for all k, where \(\{\psi _k\}\) is an orthonormal-basis of the subspace Q(B) in \(N=\ker A^{*}\).
Let \(-\,\Delta \psi + \psi =0\) and put \(\psi (r, \theta )=R(r) \Theta (\theta )\), where \(x =(r,\theta )\) are polar co-ordinates. Then since
we get
and \(\dfrac{\Theta ''}{\Theta }=-n^2\) with constant n and \(\Theta (0)=\Theta (2\pi )\); thus \( \Theta _n(\theta ) = e^{in\theta }, n\in \mathbb {Z}\) and we seek the \(L^2(0,1;rdr)\) solutions of
These solutions are given by the modified Bessel functions \(I_n(r)\) and \(K_n(r)\).
For \( n \ge 1\), \(K_n(r)\) does not lie in \(L^2(0,1;rdr)\). The function \(K_0(r)\) has a logarithmic singularity at 0, which means that \(\Delta K_0\) is not zero in the sense of distributions, excluding \(K_0\) from N. Therefore
is a basis for N; note that \(I_{-k}=I_k\).
For \(k\in \mathbb {Z}\)
and since \(v_F\in D(A_F)\), we have \(\nu _1 v_F=\frac{\partial v_F}{\partial \nu }\). On expanding \(v_F\) in \(\theta \) in terms of its Fourier series,
we derive
Consequently \(v=v_F+w\in D(A_B)\) if and only if for all \(k\in \mathbb {Z}\)
Remark 3.9
-
1.
For the Krein–von Neumann extension, \(v=v_F+w\in D(A_K)\) if and only if for all \(k\in \mathbb {Z}\) we have
$$\begin{aligned} 0 = 2\pi \frac{\partial v_{F,k}}{\partial r}(1) I_k(1). \end{aligned}$$As \(I_k(1)\ne 0\) for all \(k\in \mathbb {Z}\), this implies that
$$\begin{aligned} v_F(1,\theta ) = \frac{\partial v_{F}}{\partial r}(1,\theta ) = 0 \end{aligned}$$and hence \(v_F\in D(A)\). As there are no restrictions on w, we get \(D(A_K)= D(A) + N\), as expected. Also, the boundary condition satisfied by any \(u \in D(A_K)\) is \( \Gamma _1 u = 0\), where \( \Gamma _1 = \nu _1 - P_{\gamma _0, \nu _1} \gamma _0 \) is the regularised Neumann operator.
-
2.
For the Friedrichs extension, we formally have \(b_{jk}=\infty \) for all j, k in (3.19). This implies that w must be orthogonal to all the \(\psi _k\). As \(w\in N\), this gives \(w=0\).
4 Aharonov–Bohm Operator
Let \(\Omega = \{x{:}\,|x| <1\} {\setminus } \{0\} \subset \mathbb {R}^2\), and let A be the closure in \(L^2(\Omega )\) of \(A'\upharpoonleft _{C_0^{\infty }(\Omega )}\), where
Here, the Aharonov–Bohm magnetic potential
where \(x=(r \cos ~\theta , r\sin ~\theta )\) in polar co-ordinates and \(e_{\theta } = (-\sin ~\theta , \cos ~\theta )\) is the unit vector orthogonal to \(e_r = x/r\). Then
For \(u \in C_0^{\infty }(\Omega )\) we have
The sequence \(\{\varphi _k(\theta ): k \in \mathbb {Z}\}\), where \(\varphi _k(\theta ) = \frac{e^{-ik\theta }}{\sqrt{2\pi }}\), is an orthonormal basis for \(L^2(0,2\pi )\) and hence any \(u \in L^2(\Omega )\) has the representation
where
On substituting in (4.3), we have, with \(\lambda _k=k+\alpha \)
Since \( \min \{|\lambda _k|/r{:}\,k \in \mathbb {Z},\ 0<r<1 \} \ge \min \{\alpha ,1-\alpha \} >0\), it follows that A is strictly positive and its form domain \(Q(A_F)\) is the completion of \(C_0^{\infty }(\Omega )\) with respect to the norm given by the square root of
Let B be a positive self-adjoint operator acting in a closed subspace \(N_B\) of \(N= \text {ker}~A^{*}\) which is associated with the self-adjoint extension \(A_B\) of A in the KVB theory, and let \(a_B[\cdot ,\cdot ], a_F[\cdot ,\cdot ], b[\cdot ,\cdot ]\) be the forms of \(A_B, A_F, B\), respectively.
For \(u, \varphi \in Q(A_B)\), we have
and since \(v(R,\theta ) = \vartheta (R,\theta ) = 0\) (which follows from the definition of \(Q(A_F)\)),
Remark 4.1
Since \(v(1, \theta ) =0\) for any \(v \in Q(A_F)\), \(Q(A_F)\) coincides with Brasche and Melgaard’s form domain of \(A_F\) in [4], and so \(A_F\) is determined in their Theorem 4.5.
We now proceed as in the proof of Theorem 2.2. For \(u=u_F+w \in D(A_B)\) and \( \varphi = \vartheta + \zeta \in Q(A_B)\)
and
Consequently
If \( \{\psi _k\}\) is an orthonormal basis of Q(B), then we have with the same notation as in Sect. 3, that \(u = u_F + w \in D(A_B)\) if and only if
The transformation
is a unitary operator from \(L^2(0,1;rdr)\) onto \( L^2(0,1)\), and as \(\{e^{im\theta }/\sqrt{2 \pi }\}_{m \in \mathbb {Z}}\) is an orthonormal basis of \(L^2(\mathbb {S}^1)\) we have
In terms of this decomposition it follows that
where \(T^{(m)}\) is the closure in \(L^2(0,1)\) of the operator defined on \(C_0^{\infty }(0,1)\) by the Sturm–Liouville expression
i.e., \(T^{(m)}\) is the minimal operator in \(L^2(0,1)\) generated by \(\tau ^m\). With \(\nu = m+\alpha \), the set \(\{r^{1/2+\nu }, r^{1/2-\nu }\}\) is a fundamental system for \( \tau ^m u=0\). The expression \(\tau ^m\) is non-oscillatory. For \(m= -1,~0\), it is in the limit-circle case at 0; for all other values of m, it is in the limit-point case at 0. It is regular at 1 for all values of m. Thus \(T^{(m)}\) has deficiency indices (2, 2) for \( m=-1,~0\) and (1, 1) otherwise. We shall now apply results from [5] to determine the positive self-adjoint extensions of \(T^{(m)}\) in \(L^2(0,1)\) for all \(m \in \mathbb {Z}\). Note that the singular point here is at the left endpoint of the interval [0, 1], i.e., it is the point 0, unlike the analysis of [5], where it is at the right endpoint. If \(S^{(m)}\) is one such extension, then
is a positive self-adjoint extension of A.
Remark 4.2
We note that it is unlikely that all positive self-extensions of A are obtained in this way. This assertion is based on the situation for \(A_0 = -\Delta +1\) from Example 3.8. As in (4.11),
where now, \(T_{(m)}\) is the minimal operator generated by
At 0, \( \tau _m\) is non-oscillatory and in the limit-circle case for \(m=0\) and is otherwise limit-point. As above for A, if \(S_{(m)}\) is a positive self-adjoint extension of \(T_{(m)}\) then
is a positive self-adjoint extension of \(A_0\). All such extensions have boundary conditions which depend on behaviour at 0, in view of the presence of the extension \(S_{(0)}\) of \(T_{(0)}\) which has deficiency indices (1, 1). However in Remark 3.9 we saw that this is not so for the Krein–von Neumann extension of \(A_0!\)
We shall proceed to determine the extensions \(T^{(m)}\) in (4.11).
4.1 The Case when \(\tau ^m\) is Limit Point at 0 (\(m \ne -1,0\))
Theorem 2.1 in [5] establishes a one-one correspondence between the positive self-adjoint extensions of \(T^{(m)}\) in this case and the one-parameter family \(\{T^{(m)}_l\}\), \(0 \le l \le \infty \) of restrictions of \((T^{(m)})^{*}\) to the domains
Here \(\psi \) is a real function in \(L^2(0,1)\) which satisfies \(\tau ^m \psi =0\) and \( \psi (1) =1.\) We therefore have
4.2 The Case when \(\tau ^m\) is Limit-Circle at 0 (\(m=-1,0\)) and \(\mathbf dim ~N_B =1\)
From Theorem 2.2 in [5] and writing \(T^{*}\) instead of \(\left( T^{(m)}\right) ^{*}\) for simplicity, it follows that the positive self-adjoint extensions of the operator \(T^{(m)}\) which correspond to the pair \(\{B,N_B\}\) in the KVB theory with \(\text {dim}N_B =1\) form a one-parameter family \(T_{\beta }\) of restrictions of \(T^{*}\) with domains
where \(\psi \) is a real function in \(N_B\) with \(\psi (1) =1\), g is the non-principal solution of \(\tau ^m u=0\) and \(\beta \ge 0\). The non-principal solution is \(r^{1/2 - |\nu |},~ \nu = m+\alpha \). The Wronskian W is given by
The limits at 0 of the first and the second terms in (4.17) exist separately. To see this, let
Hence by the Jacobi identity [5, Equation (1.10)], for \(v\in \mathcal {D}(T^{*})\) we get
Thus, since \( g \in L^2(0,1)\),
which implies that
both exist. From [9, Remark 3] (see also [5, (2.9)]), \(\lim _{r \rightarrow 0+}(v/g)(r) \) exists, which confirms our assertion that the separate limits exist.
We shall now determine the boundary conditions satisfied by the self-adjoint extensions of \(T^{(m)}\) in the two cases corresponding to \( \nu = m + \alpha ,\ m = -1, 0,\ \alpha \in (0,1)\).
4.2.1 The Case \(m = -1,\ \nu =-1 +\alpha \in (-1,0)\)
In this case, the non-principal solution is \( g(r) = r^{1/2 +\nu }\) and \(\psi (r) = \gamma \left( C_1 r^{1/2-\nu } + C_2 r^{1/2+\nu }\right) \), where \( \gamma = (C_1+C_2)^{-1}\) for \(C_1,C_2\) are constants and \(C_1\ne 0\). Thus,
and so using (4.18)
The value at \(r=1\) is
By (4.20) and since \( \nu < 0\), the limits at 0 of both terms in (4.21) exist and
Thus the boundary condition for \(A_B\) in this case is
where \( f_1(r) := \left[ r^{1/2+\nu } v'(r) -(1/2+\nu ) r^{-1/2 +\nu } v(r)\right] \).
4.2.2 The Case \(m =0,\ \nu = \alpha \in (0,1)\)
This time, the non-principal solution is \(g(r) = r^{1/2-\nu }\) and, with \(\psi \) as above, we have
giving
Therefore
By (4.25) and since \( \nu >0\), both limits at 0 in (4.26) exist and
Thus the boundary condition in this case is
where \( f_2(r) := \left[ r^{1/2-\nu } v'(r) -(1/2-\nu ) r^{-1/2-\nu }v(r)\right] \).
4.3 The Case when \(\tau ^m\) is Limit-Circle at 0 (\(m=-1,0\)) and \(\hbox {dim}N_B=2\)
From [5, Theorem 2.2], we have
where \(B:= (b_{jk})_{j,k=1,2}\) is a matrix of parameters which is non-negative, \(\{\psi _1, \psi _2\}\) is a real orthonormal basis of \(N_B\) and \(c_1,~c_2\) are determined by the values of v / g at 0 and 1:
The main difference from the analysis of the previous section is that we now replace \(\psi \) by an orthonormal basis \((\psi _1,\psi _2)\) obtained from the linearly independent basis elements
On using the Gram–Schmidt procedure, we obtain the orthogonal vectors
and the orthonormal system
The non-principal solution is \(g(r) = r^{1/2-| \nu |}\) and we have
and
Let
Then
and we set
Also
giving
We also have
Setting \(V=\Psi c\), where
is invertible and has inverse
The boundary condition in (4.30) is therefore
For the Krein–von Neumann extension \(\left( T^{(m)}\right) _K\) of the one-dimensional operator, the boundary condition is determined by \(\Theta _1 = \Theta _2 = 0\): Hence
and
Following Remark 2.1, the Friedrichs extension \(\left( T^{(m)}\right) _F\) is obtained by \(c_1=c_2=0\), so that the right hand side of (4.30) is finite. Thus, from (4.31), the boundary conditions are given by \((v/g)(0)=(v/g)(1)=0\), i.e.,
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Brown, B.M., Evans, W.D. & Wood, I.G. Positive Self-adjoint Operator Extensions with Applications to Differential Operators. Integr. Equ. Oper. Theory 91, 41 (2019). https://doi.org/10.1007/s00020-019-2540-4
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DOI: https://doi.org/10.1007/s00020-019-2540-4