On connection of asymptotic formulas for the counting function and for the characteristic numbers of a compact positive operator
V. I. Voytitsky V. I. Vernadsky Crimean Federal University, Simferopol
Abstract:
Let operator
$G$ be compact positive operator acting in separable Hilbert space. According with theorem of Hilbert-Schmidt its characteristic numbers
$\mu_n$ are positive finite multiple with unique limit point at infinity. In spectral problems of mathematical physics such numbers, as a rule, have power (Weyl's) asymptotic. Sometimes it is more convenient to use asymptotic of counting function
$N(r)$ that is equal to number (taking into account the multiplicity) of characteristic numbers
$\mu_n$ in the interval
$(0; r).$ For single eigenvalues recalculation of asymptotic formulas is a simple exercise. We prove several theorems on connection between asymptotic of
$\mu_n$ and
$N(r)$ for an arbitrary compact positive operator
$G$.
Theorem 1. If
$\mu_n = a n^{\alpha}(1+o(1)),\ n \to \infty$, where
$\alpha>0$, then
\begin{equation*}
N(r) = a^{-1/\alpha} r^{1/\alpha}(1+o(1)), \quad r \to +\infty.
\end{equation*}
Theorem 2. If $N(r)= a r^{\alpha}(1+o(1)),\ r \to +\infty, \ \alpha>0,$ then
$$\mu_n = a^{-1/\alpha} n^{1/\alpha}(1+o(1)), \ n \to \infty.$$
Theorem 3. If $\mu_n = a n^{\alpha} + O(n^{\beta}),\ n \to \infty$, where
$\alpha>\beta \geq \alpha-1, \quad \alpha>0,$ then
$$
N(r) = a^{-1/\alpha} r^{1/\alpha} + O(r^{\frac{1+\beta-\alpha}{\alpha}}), \quad r \to +\infty.
$$
Theorem 4. If $N(r)= a r^{\alpha} + O(r^{\beta}),\ r \to +\infty,$ where
$\alpha>\beta \geq 0$, then
$$\mu_n = a^{-1/\alpha} n^{1/\alpha} + O(n^{\frac{1+\beta-\alpha}{\alpha}}), \quad n \to \infty.$$
As an application we study asymptotic of a diagonal operator-matrix $\mathcal{A} = \begin{pmatrix} A & 0 \\ 0 & B \end{pmatrix}$ if it is known the power asymptotic of operators
$A$ and
$B$.
Keywords:
compact operator, infinitely large sequence, subsequence, power asymptotic, Landau symbols.
UDC:
517.98,
517.15
MSC: 47A10,
26A12
DOI:
10.37279/1729-3901-2021-20-2-12-23