  Boman and Hendrickson observed that one can solve linear systems in Laplacian
matrices in time $\bigO{m^{3/2 + o (1)} \ln (1/\epsilon)}$ by preconditioning
with the Laplacian of a low-stretch spanning tree. By examining the
distribution of eigenvalues of the preconditioned linear system, we prove that
the preconditioned conjugate gradient will actually solve the linear system in
time $\softO{m^{4/3} \ln (1/\epsilon)}$.
