let M1 be Matrix of n,K; :: thesis: ( M1 = M @ implies M1 is central_symmetric )
assume A1: M1 = M @ ; :: thesis: M1 is central_symmetric
A2: Indices M = [:(Seg n),(Seg n):] by MATRIX_0:24;
let i, j, k, l be Nat; :: according to MATRIX17:def 3 :: thesis: ( [i,j] in Indices M1 & k = (n + 1) - i & l = (n + 1) - j implies M1 * (i,j) = M1 * (k,l) )
assume that
A3: [i,j] in Indices M1 and
A4: ( k = (n + 1) - i & l = (n + 1) - j ) ; :: thesis: M1 * (i,j) = M1 * (k,l)
[i,j] in [:(Seg n),(Seg n):] by A3, MATRIX_0:24;
then A5: ( i in Seg n & j in Seg n ) by ZFMISC_1:87;
then A6: [j,i] in [:(Seg n),(Seg n):] by ZFMISC_1:87;
( (n + 1) - j in Seg n & (n + 1) - i in Seg n ) by A5, Lm1;
then A7: ( [((n + 1) - i),((n + 1) - j)] in [:(Seg n),(Seg n):] & [((n + 1) - j),((n + 1) - i)] in [:(Seg n),(Seg n):] ) by ZFMISC_1:87;
thus M1 * (i,j) = M * (j,i) by A1, A2, A6, MATRIX_0:def 6
.= M * (l,k) by A4, A6, A2, Def3
.= M1 * (k,l) by A1, A7, A2, A4, MATRIX_0:def 6 ; :: thesis: verum