let n be Nat; :: thesis: ( not 3 divides n iff ex k being Nat st
( n = (3 * k) + 1 or n = (3 * k) + 2 ) )
consider K being Nat such that
A1: ( n = 3 * K or n = (3 * K) + 1 or n = (3 * K) + 2 ) by Th23;
thus ( not 3 divides n implies ex k being Nat st
( n = (3 * k) + 1 or n = (3 * k) + 2 ) ) by A1; :: thesis: ( ex k being Nat st
( n = (3 * k) + 1 or n = (3 * k) + 2 ) implies not 3 divides n )
given k being Nat such that A2: ( n = (3 * k) + 1 or n = (3 * k) + 2 ) ; :: thesis: not 3 divides n
given t being Nat such that A3: n = 3 * t ; :: according to NAT_D:def 3 :: thesis: contradiction

per cases ( n = (3 * k) + 1 or n = (3 * k) + 2 ) by A2;

suppose n = (3 * k) + 1 ; :: thesis: contradiction

then 1 / 3 = t - k by A3;
hence contradiction by NAT_D:33; :: thesis: verum

end;

suppose n = (3 * k) + 2 ; :: thesis: contradiction

then 2 / 3 = t - k by A3;
hence contradiction by NAT_D:33; :: thesis: verum

end;

end;