reserve c for Complex;
reserve r for Real;
reserve m,n for Nat;
reserve f for complex-valued Function;

theorem Th13:
  ( #Z n) `| = n (#) #Z (n-1)
  proof
    n in NAT by ORDINAL1:def 12;
    hence ( #Z n) `| = (n(#) #Z (n-1)) | [#]REAL by HFDIFF_1:28
    .= n(#) #Z (n-1);
  end;
