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Homework Help: Commutative Ring question

  1. Dec 2, 2005 #1
    I have this question, just curious if what I did is correct.
    Let A and R be rings, and let f:A->R be a function satisfying:

    f(a+b) = f(a)+f(b) and f(ab) = f(a)f(b).

    Prove that if f is surjective, then f(1) = 1.

    Note: I will use * for multiplication for clarity where things would have just otherwise been next to each other.
    Also, note that A and R are commutative rings, both of which have a multiplicative identity 1.

    First, R is a ring, so 1 is in R. Also, because this function is surjective, there must be some a in A, that satisfies f(a) = 1. So, we can rewrite f(a) = 1 as: 1 = f(a*1) = f(a)f(1) = 1*f(1) [because f(a) = 1] = f(1) = 1.

    That looks sufficient to me, but I don't like it when I don't use something I could have (ie, the + part of the function). Does this look correct? Thanks.
  2. jcsd
  3. Dec 2, 2005 #2


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    Yes, but you have 1 = .... = f(1) = 1. You shouldn't have that underlined part.
  4. Dec 2, 2005 #3
    Thanks. You are right, I cannot say that f(1) = 1 in that sense.
  5. Dec 2, 2005 #4


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    Yes, but of course you don't need to. You have 1 = ... = f(1), and that's all you needed.
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