let OAS be OAffinSpace; :: thesis: for f being Permutation of the carrier of OAS st f is dilatation holds
( ( f = id the carrier of OAS or for x being Element of OAS holds f . x <> x ) iff for x, y being Element of OAS holds x,f . x '||' y,f . y )

let f be Permutation of the carrier of OAS; :: thesis: ( f is dilatation implies ( ( f = id the carrier of OAS or for x being Element of OAS holds f . x <> x ) iff for x, y being Element of OAS holds x,f . x '||' y,f . y ) )
assume A1: f is dilatation ; :: thesis: ( ( f = id the carrier of OAS or for x being Element of OAS holds f . x <> x ) iff for x, y being Element of OAS holds x,f . x '||' y,f . y )
A2: now :: thesis: ( ( for x, y being Element of OAS holds x,f . x '||' y,f . y ) & f <> id the carrier of OAS implies for x being Element of OAS holds not f . x = x )
assume A3: for x, y being Element of OAS holds x,f . x '||' y,f . y ; :: thesis: ( f <> id the carrier of OAS implies for x being Element of OAS holds not f . x = x )
assume f <> id the carrier of OAS ; :: thesis: for x being Element of OAS holds not f . x = x
then consider y being Element of OAS such that
A4: f . y <> (id the carrier of OAS) . y by FUNCT_2:63;
given x being Element of OAS such that A5: f . x = x ; :: thesis: contradiction
x <> y by A5, A4;
then consider z being Element of OAS such that
A6: not x,y,z are_collinear by DIRAF:37;
x,z '||' f . x,f . z by ;
then x,z,f . z are_collinear by ;
then A7: z,f . z,x are_collinear by DIRAF:30;
z,f . z,z are_collinear by DIRAF:31;
then A8: z,f . z '||' x,z by ;
A9: f . y <> y by A4;
x,y '||' f . x,f . y by ;
then A10: x,y,f . y are_collinear by ;
then y,x,f . y are_collinear by DIRAF:30;
then A11: y,x '||' y,f . y by DIRAF:def 5;
A12: y,f . y,x are_collinear by ;
A13: now :: thesis: not z = f . zend;
y,f . y '||' z,f . z by A3;
then y,f . y '||' x,z by ;
then y,x '||' x,z by ;
then x,y '||' x,z by DIRAF:22;
hence contradiction by A6, DIRAF:def 5; :: thesis: verum
end;
now :: thesis: ( ( f = id the carrier of OAS or for z being Element of OAS holds f . z <> z ) implies for x, y being Element of OAS holds x,f . x '||' y,f . y )
assume A14: ( f = id the carrier of OAS or for z being Element of OAS holds f . z <> z ) ; :: thesis: for x, y being Element of OAS holds x,f . x '||' y,f . y
let x, y be Element of OAS; :: thesis: x,f . x '||' y,f . y
A15: x,y '||' f . x,f . y by ;
A16: now :: thesis: ( ( for z being Element of OAS holds f . z <> z ) implies x,f . x '||' y,f . y )
assume A17: for z being Element of OAS holds f . z <> z ; :: thesis: x,f . x '||' y,f . y
assume A18: not x,f . x '||' y,f . y ; :: thesis: contradiction
then consider z being Element of OAS such that
A19: x,f . x,z are_collinear and
A20: y,f . y,z are_collinear by ;
set t = f . z;
x,f . x,f . z are_collinear by ;
then A21: x,f . x '||' z,f . z by ;
y,f . y,f . z are_collinear by ;
then A22: y,f . y '||' z,f . z by ;
z <> f . z by A17;
hence contradiction by A18, A21, A22, DIRAF:23; :: thesis: verum
end;
( f = id the carrier of OAS implies x,f . x '||' y,f . y ) by DIRAF:20;
hence x,f . x '||' y,f . y by ; :: thesis: verum
end;
hence ( ( f = id the carrier of OAS or for x being Element of OAS holds f . x <> x ) iff for x, y being Element of OAS holds x,f . x '||' y,f . y ) by A2; :: thesis: verum