We give a dichotomy theorem for the problem of counting homomorphisms to
directed acyclic graphs. $H$ is a fixed directed acyclic graph.
The problem is, given an input digraph $G$, how many homomorphisms are there
from $G$ to $H$. We give a graph-theoretic classification, showing that
for some digraphs $H$, ... more >>>

We study the complexity of finding the values and optimal strategies of
MEAN PAYOFF GAMES on graphs, a family of perfect information games
introduced by Ehrenfeucht and Mycielski and considered by Gurvich,
Karzanov and Khachiyan. We describe a pseudo-polynomial time algorithm
for the solution of such games, the decision ... more >>>

The model of span programs is a linear algebraic model of
computation. Lower bounds for span programs imply lower bounds for
contact schemes, symmetric branching programs and for formula size.
Monotone span programs correspond also to linear secret-sharing schemes.
We present a new technique for proving lower bounds for ... more >>>