The Q-matrix of a connected graph G = (V,E) is Q = (q ∂(x,y))x,yεv, where ∂(x,y) is the graph distance. Let q(G] be the range of q ε (-1,1) for which the Q-matrix is strictly positive. We obtain a sufficient condition for the equality q(G̃) = q(G) where g̃ is an extension of a finite graph G by joining a square. Some concrete examples are discussed.
- Markov sum
- Positive definite kernel