We consider the mapping
that assigns to a four-tuple ( a , b , c , d ) {\displaystyle {}(a,b,c,d)} the four-tuple
Find an example of a four-tuple ( a , b , c , d ) {\displaystyle {}(a,b,c,d)} with the property that all iterations Ψ n ( a , b , c , d ) {\displaystyle {}\Psi ^{n}(a,b,c,d)} for n ≤ 25 {\displaystyle {}n\leq 25} do not yield the zero-tuple. Check your result on http://www.vier-zahlen.bplaced.net/raetsel.php .