Integers/prime factorization question

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  1. gangsta316's Avatar
    • Overlord in Training
    • Location: United Kingdom
    • Posts: 2,851
    Integers/prime factorization question
    How would I find all solutions to an equation like
    x^2 = y^3
    for integers x and y.

    This is what I did:
    x = y^(3/2)
    So y needs to be a perfect square for x to be an integer. So solution should be
    (x, y) = (t^3, t^2) for any integer t.
    Is that all there is? Is that the proper way of solving such an equation? I've been told that there's a proper method where you use prime factorization, but I can't see it. I know that all the powers of the primes on the LHS will be multiples of 2, and all the powers of the primes on the RHS will be multiples of 3. But what can we do with that?
  2. Unbounded's Avatar
    • TSR Demigod
    Re: Integers/prime factorization question
    I suppose something like the following might be a little more rigorous; by the fundamental theorem of arithmetic, we can write:  x = p_1^{a_1}p_2^{a_2} \cdots p_m^{a_m}, y = q_1^{b_1}q_2^{b_2} \cdots q_n^{b_n} , where  p_i, q_i are distinct prime divisors of x and y respectively, and  a_i, b_i are positive integers. So,

     p_1^{2a_1} p_2^{2a_2} \cdots p_m^{2a_m} = q_1^{3b_1}q_2^{3b_2} \cdots q_n^{3b_n}

    Therefore  p_i^{2a_i} | q_1^{3b_1}q_2^{3b_2} \cdots q_n^{3b_n} \iff p_i^{2a_i} | q_j^{3b_j} from the definition of a prime, for some j.

    At the same time,  q_j^{3b_j} |p_1^{2a_1} p_2^{2a_2} \cdots p_m^{2a_m} \iff q_j^{3b_j} | p_k^{2a_k}

    Since pairs of in each prime decomposition are coprime, clearly,  p_i = q_j, 2a_i = 3b_j for some (and each) i and j. And m = n.
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