let n be Nat; :: thesis: for K being Field
for M1, M2 being Matrix of n,K st M2 is invertible & M1 is_congruent_Matrix_of M2 & n > 0 holds
M1 is invertible

let K be Field; :: thesis: for M1, M2 being Matrix of n,K st M2 is invertible & M1 is_congruent_Matrix_of M2 & n > 0 holds
M1 is invertible

let M1, M2 be Matrix of n,K; :: thesis: ( M2 is invertible & M1 is_congruent_Matrix_of M2 & n > 0 implies M1 is invertible )
assume that
A1: M2 is invertible and
A2: M1 is_congruent_Matrix_of M2 and
A3: n > 0 ; :: thesis: M1 is invertible
consider M4 being Matrix of n,K such that
A4: M4 is invertible and
A5: M1 = ((M4 @) * M2) * M4 by A2;
set M6 = (M4 ~) @ ;
set M5 = M4 @ ;
A6: ( width M4 = n & width (M4 ~) = n ) by MATRIX_0:24;
len M4 = n by MATRIX_0:24;
then A7: ((M4 ~) * M4) @ = (M4 @) * ((M4 ~) @) by ;
A8: M4 ~ is_reverse_of M4 by ;
then (M4 ~) * M4 = 1. (K,n) by MATRIX_6:def 2;
then A9: (M4 @) * ((M4 ~) @) = 1. (K,n) by ;
len (M4 ~) = n by MATRIX_0:24;
then (M4 * (M4 ~)) @ = ((M4 ~) @) * (M4 @) by ;
then (M4 @) * ((M4 ~) @) = ((M4 ~) @) * (M4 @) by ;
then M4 @ is_reverse_of (M4 ~) @ by ;
then M4 @ is invertible by MATRIX_6:def 3;
then (M4 @) * M2 is invertible by ;
hence M1 is invertible by ; :: thesis: verum