He showed that it's a homomorphism. An isomorphism is just a bijective homomorphism, no? Of course, his function isn't even close to bijective (it's not onto)

He showed that it's a homomorphism. An isomorphism is just a bijective homomorphism, no? Of course, his function isn't even close to bijective (it's not onto), so it's just a homomorphism, not an isomorphism.