let K be Field; :: thesis: for n, m, k being Element of NAT
for M1 being Matrix of the carrier of K,n,
for M2 being Matrix of the carrier of K,m, st width M1 = len M2 & 0 < len M1 & 0 < len M2 holds
M1 * M2 is Matrix of the carrier of K,n,
let n, m, k be Element of NAT ; :: thesis: for M1 being Matrix of the carrier of K,n,
for M2 being Matrix of the carrier of K,m, st width M1 = len M2 & 0 < len M1 & 0 < len M2 holds
M1 * M2 is Matrix of the carrier of K,n,
let M1 be Matrix of the carrier of K,n,; :: thesis: for M2 being Matrix of the carrier of K,m, st width M1 = len M2 & 0 < len M1 & 0 < len M2 holds
M1 * M2 is Matrix of the carrier of K,n,
let M2 be Matrix of the carrier of K,m,; :: thesis: ( width M1 = len M2 & 0 < len M1 & 0 < len M2 implies M1 * M2 is Matrix of the carrier of K,n, )
assume that
A1: width M1 = len M2 and
A2: 0 < len M1 and
A3: 0 < len M2 ; :: thesis: M1 * M2 is Matrix of the carrier of K,n,
width M1 = m by A1, MATRIX_1:def 3;
then A4: ( len M1 = n & width M2 = k ) by A1, A3, MATRIX_1:20, MATRIX_1:def 3;
( len (M1 * M2) = len M1 & width (M1 * M2) = width M2 ) by A1, MATRIX_3:def 4;
hence M1 * M2 is Matrix of the carrier of K,n, by A2, A4, MATRIX_1:20; :: thesis: verum