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Homework Help: Distinct primes

  1. Aug 30, 2007 #1
    1. The problem statement, all variables and given/known data
    Let p and q be distinct primes. Suppose that H is proper subset of the integers nd H is grou under addition that contains exactly three elements of the set {p,p+q,pq, p^q, q^p}. Determine which of the following are the three elements in H:
    a) pq, p^q, q^p
    b)p+q, pq,q^p
    c) p, p+q,pq
    d) p, p^q,q^p
    e)p,pq, p^q

    2. Relevant equations

    no equations for this problem
    3. The attempt at a solution
    The back of my textbook says the answer is e, but I thought it would be c . I don't understand why the answer is e because if a group is under addition, the additive properties of the group should be p+q . the properties for a group under multiplication would be p*q.
  2. jcsd
  3. Aug 30, 2007 #2

    matt grime

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    Science Advisor
    Homework Helper

    p*q ia p added to itself q times. So obviously if p is in a subgroup of the integers with addition, then so is p*q.

    It can't be c. If it were c, then it would contain co-prime integers, hence all integers, but you are told it is a *proper* subgroup.
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