On the kernel of tree incidence matrices

Abstract

We study the height of the delta peak at 0 in the spectrum of random tree incidence matrices. We show that the average fraction of the spectrum occupied by the eigenvalue 0 in a large random tree is asymptotic to 2x-1 = 0.1342865808195677459999... where x is the unique real root of x = exp(-x). For finite trees, we give a closed form, a generating function, and an asymptotic estimate for the sequence 1,0,3,8,135,1164,21035,.... of the total multiplicity of the eigenvalue 0 in the set of n^{n-2} tree incidence matrices of size n>0.