
theorem Th11: :: Even101
  for n being even Nat st n<=3 holds n=0 or n=2
proof
  let n be even Nat such that
A1: n<=3;
  n<>2*1+1;
  then n<2+1 by A1,XXREAL_0:1;
  then
A2: n<=2 by NAT_1:13;
  per cases by A2,XXREAL_0:1;
  suppose
    n=2;
    hence thesis;
  end;
  suppose
    n<1+1;
    then n<=1 by NAT_1:13;
    hence thesis by Th10;
  end;
end;
