Included among these is the sum of the squares of the characteristic numbers of P1, i.e., the sum of ihe characteristic numbers of N1 = AA*; this is the well-known unitary invariant Eapdpq of Frobenius. p,q When A is normal AA* = A*A or PU.UI*P1 = U*"P.P,U so that I= U*PU = (U*PU)2. Hence P1 = U*P,U or UP1 = POU. Conversely if UP1 = P1U we have A*A = A*A so that a matrix A is normal. when and o...