We consider compact metric graphs with an arbitrary self adjoint realisation of the differential Laplacian. After discussing spectral properties of Laplacians, we prove several versions of trace formulae, relating Laplace spectra to sums over periodic orbi
rstcaseKostrykinandSchradershowedthatthenumberofnegativeLaplaceeigenvaluesis nite;infact,itsnumberisboundedbythenumberofpositiveeigenvaluesofL[KS06b].
Thatimplies,inparticular,thatLaplacianswithnon-Robinboundaryconditions,whereL=0,possessanon-negativespectrum.Moreover,theyfoundthefollowinglowerbound,
≥ s2.
Heres≥0istheuniquesolutionof
stanh slmin(4.5)
λ+ik
Eventually,oneobtains
F(k)=1 e2ikl
0eeikl0λ ikikl .2=λ.
Thisconditionisequivalentto(4.6),demonstratingthatthebound(4.5)issharpforthis‘quantumgraph’.
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