Z[√2]={a+b√2 | a,b apartin Z}
f:Z[√2] --> Z, f(a+b√2) = [tex] a^{2} - 2b^{2}[/tex]
A={x apartine Z[√2] | f(x) = -1}

a) A =/= multimii vide
b) f(xy) = f(x) · f(y)