@[USERNAME]But how do we know B is invertable? I found a proof that does not assume B is invertable:Suppose we have x such that ABx = lambda * x. Left multiply both sides by B: BABx = lambda * Bx. This shows that Bx is an eigen vector of BA, and its eigen value is lambda.####[['B', 'Mathematical_Related_Concept', 'NEU'], ['x', 'Mathematical_Related_Concept', 'NEU']]