Hi, I am really struggling with a quetion about ideals on my rings, polynomials and fields paper. The question is:

Show that the set

I={a+b(-5)^1/2:a,b in Z and a-b is even}

Is an ideal in Z[-5]^1/2

I do not have a lot of knowledge on Ideals soa simple explanation would be great. Would appreciate any help on Ideals at all.