
theorem Th7: :: Odd101
  for n being odd Nat st n<=4 holds n=1 or n=3
proof
  let n be odd Nat such that
A1: n<=4;
  n<>2*2;
  then n<3+1 by A1,XXREAL_0:1;
  then
A2: n<=3 by NAT_1:13;
  per cases by A2,XXREAL_0:1;
  suppose
    n=3;
    hence thesis;
  end;
  suppose
    n<2+1;
    then n<=2 by NAT_1:13;
    hence thesis by Th6;
  end;
end;
