Tutte polynomial

The Tutte polynomial $T_M(x,y) = \sum_{A \subseteq E} (x-1)^{r(E)-r(A)}\,(y-1)^{|A|-r(A)}$, where $r$ is the rank function. It is the universal deletion–contraction invariant. For example, $T_M(1,1)$ is the number of bases, $T_M(2,2) = 2^n$, and the characteristic polynomial of $M$ is $(-1)^{r}\,T_M(1-\lambda, 0)$. Copied from polyDB (TUTTE_POLYNOMIAL).