let n be Nat; :: thesis: for K being Field
for M1, M2 being Matrix of n,K st n > 0 & M1 * M2 = 0. (K,n,n) & M1 is invertible holds
M1 commutes_with M2
let K be Field; :: thesis: for M1, M2 being Matrix of n,K st n > 0 & M1 * M2 = 0. (K,n,n) & M1 is invertible holds
M1 commutes_with M2
let M1, M2 be Matrix of n,K; :: thesis: ( n > 0 & M1 * M2 = 0. (K,n,n) & M1 is invertible implies M1 commutes_with M2 )
assume that
A1: n > 0 and
A2: M1 * M2 = 0. (K,n,n) and
A3: M1 is invertible ; :: thesis: M1 commutes_with M2
A4: M1 ~ is_reverse_of M1 by A3, Def4;
A5: len M2 = n by MATRIX_0:24;
A6: ( len M1 = n & width M1 = n ) by MATRIX_0:24;
A7: len (M1 ~) = n by MATRIX_0:24;
A8: width (M1 ~) = n by MATRIX_0:24;
M2 = (1. (K,n)) * M2 by MATRIX_3:18
.= ((M1 ~) * M1) * M2 by A4
.= (M1 ~) * (0. (K,n,n)) by A2, A6, A5, A8, MATRIX_3:33
.= 0. (K,n,n) by A1, A7, A8, Th2 ;
hence M1 commutes_with M2 by Th3; :: thesis: verum