 Algebraic stability of a Runge-Kutta method

A Runge-Kutta method is algebraically stable when $b_i\ge 0$ and the matrix $M$ with $M_{ij}=b_i{a}_{ij}+b_j{a}_{ji}-b_i{b}_j$ is positive semidefinite. Algebraic stability implies contractivity for dissipative differential equations.