let n be positive Integer; :: thesis: ( n + 1 divides (n ^2) + 1 iff n = 1 )
thus ( n + 1 divides (n ^2) + 1 implies n = 1 ) :: thesis: ( n = 1 implies n + 1 divides (n ^2) + 1 )

proof

assume n + 1 divides (n ^2) + 1 ; :: thesis: n = 1
then n: n + 1 divides (n * (n + 1)) - (n - 1) ;
n + 1 divides n * (n + 1) by INT_1:def 3;
then n1: n + 1 divides n - 1 by n, INT_5:2;
assume o: n <> 1 ; :: thesis: contradiction
n >= 0 + 1 by INT_1:7;
then n > 1 by o, XXREAL_0:1;
then n - 1 > 1 - 1 by XREAL_1:9;
then n + 1 <= n - 1 by n1, INT_2:27;
then (n + 1) - n <= (n - 1) - n by XREAL_1:9;
hence contradiction ; :: thesis: verum

end;

thus ( n = 1 implies n + 1 divides (n ^2) + 1 ) ; :: thesis: verum