which is impossible since the right side is even. The proof that (2, 1− √ −5) 6= (1) is similar. For another proof, complex conjugation is an operation on ideals, a 7→ a := {α : α ∈ a} which respects addition and multiplication of ideals, and (α, β) = (α, β). In particular, the conjugate of (2, 1 + √ −5) is (2, 1− √ −5), so if (2, 1 + √ −5) = (1) then (2, 1− √ −5) = (1), so the product (2, 1 + ...