 reserve n,s for Nat;

theorem Th71:
  for x being number holds
    x in SquareNumbers iff x is square
  proof
    let x be number;
    thus x in SquareNumbers implies x is square;
    assume x is square; then
    consider n being Nat such that
A1: x = n ^2 by PYTHTRIP:def 3;
    x = Polygon (4,n) by A1;
    hence thesis;
  end;
