reserve m, n for Nat;

theorem
  Moebius 3 = -1
proof
  Moebius 3 = (-1) |^ card support ppf 3 by Def3,PEPIN:41
    .= (-1) |^ card {3} by Th8,PEPIN:41
    .= (-1) |^ 1 by CARD_1:30;
  hence thesis;
end;
