Remarks on counting negative eigenvalues of Schrödinger operator on regular metric trees

Abstract

We discuss estimates on the number $N_{-}(\alpha )$ of negative eigenvalues of the Schrödinger operator $-\Delta -\alpha V$ on regular metric trees, as depending on the properties of the potential $V\ge 0$ and on the value of the large parameter $\alpha$. We obtain conditions on $V$ guaranteeing the behavior $N_{-}(\alpha )=O({\alpha }^p)$ for any given $p\ge 1/2$. For a special class of trees we show that these conditions are not only sufficient but also necessary. For $p>1/2$ the order-sharp estimates involve a (quasi-)norm of $V$ in some `weak' $L_p$- or ${\ell }_p(L_1)$-space. We show that the results can be easily derived from the ones of an earlier paper by Naimark and the author, Proc. London Math. Soc. (3) 80 (2000), 690-724. The results considerably improve the estimates found in the recent paper by Ekholm, Frank, and Kovařík, arXive:0710.5500.

19 pages