let n be Nat; :: thesis: for K being Field
for M1, M2 being Matrix of n,K st M1 is Idempotent & M2 is Idempotent & M1 * M2 = - (M2 * M1) holds
M1 + M2 is Idempotent
let K be Field; :: thesis: for M1, M2 being Matrix of n,K st M1 is Idempotent & M2 is Idempotent & M1 * M2 = - (M2 * M1) holds
M1 + M2 is Idempotent
let M1, M2 be Matrix of n,K; :: thesis: ( M1 is Idempotent & M2 is Idempotent & M1 * M2 = - (M2 * M1) implies M1 + M2 is Idempotent )
assume that
A1: ( M1 is Idempotent & M2 is Idempotent ) and
A2: M1 * M2 = - (M2 * M1) ; :: thesis: M1 + M2 is Idempotent
A3: ( M1 * M1 = M1 & M2 * M2 = M2 ) by A1, Def1;

per cases ( n > 0 or n = 0 ) by NAT_1:3;

suppose A4: n > 0 ; :: thesis: M1 + M2 is Idempotent

A5: ( len (M1 * M2) = n & width (M1 * M2) = n ) by MATRIX_1:25;
A6: ( len M2 = n & width M2 = n ) by MATRIX_1:25;
A7: ( len ((M1 * M1) + (M2 * M1)) = n & width ((M1 * M1) + (M2 * M1)) = n ) by MATRIX_1:25;
A8: ( len (M2 * M1) = n & width (M2 * M1) = n ) by MATRIX_1:25;
A9: ( len (M1 * M1) = n & width (M1 * M1) = n ) by MATRIX_1:25;
A10: ( len M1 = n & width M1 = n ) by MATRIX_1:25;
( len (M1 + M2) = n & width (M1 + M2) = n ) by MATRIX_1:25;
then (M1 + M2) * (M1 + M2) = ((M1 + M2) * M1) + ((M1 + M2) * M2) by A4, A10, A6, MATRIX_4:62
.= ((M1 * M1) + (M2 * M1)) + ((M1 + M2) * M2) by A4, A10, A6, MATRIX_4:63
.= ((M1 * M1) + (M2 * M1)) + ((M1 * M2) + (M2 * M2)) by A4, A10, A6, MATRIX_4:63
.= (((M1 * M1) + (M2 * M1)) + (M1 * M2)) + (M2 * M2) by A5, A7, MATRIX_3:5
.= ((M1 * M1) + ((M2 * M1) + (- (M2 * M1)))) + (M2 * M2) by A2, A9, A8, MATRIX_3:5
.= ((M1 * M1) + (0. K,n,n)) + (M2 * M2) by A8, MATRIX_4:2
.= M1 + M2 by A3, MATRIX_3:6 ;
hence M1 + M2 is Idempotent by Def1; :: thesis: verum

end;

suppose n = 0 ; :: thesis: M1 + M2 is Idempotent

then (M1 + M2) * (M1 + M2) = M1 + M2 by MATRIX_1:36;
hence M1 + M2 is Idempotent by Def1; :: thesis: verum

end;

end;