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A quadratic question

July 8, 2006 pm31 4:13 pm

This one has been puzzling me: for ax2 + bx + c = 0, let a, b, and c be integers between -100 and 100 inclusive, a different from 0.

How many of these equations have a double root (ie, b2 – 4ac = 0).

All I can come up with is a brute force count, which I really don’t want to do. Anything clever out there?

2 Comments leave one →
  1. JBL permalink
    July 25, 2006 pm31 2:51 pm 2:51 pm

    So, two cases: if b is 0, we must have ac = 0. Since a isn’t 0,
    c must be, so that’s 200 equations we pick up there.
    Otherwise, b isn’t zero. Certainly b has to be even. So let b
    = 2k, where k runs from -50 to 50, skipping 0. Obviously, the
    sign isn’t going to matter here, so we can just take k running
    from 1 to 50 and double our answer at the end. Then we have k^2
    = ac. So, we need the number of factorizations of 1^2, 2^2,
    3^2, …, 50^2 into positive integers. Then we double that
    (because we can take negatives of both a and c) and then we
    double the result again (because I said so earlier, for the sign
    of b) and add the 200 in from the other case. Now, a
    factorization into two parts is the same as a factor, so we need
    the sum of the number of factors of the first 50 squares.
    Unfortunately, I have no clever way of approaching that problem
    at all. Mathematica tells me that the first fifty squares have
    the following numbers of factors: {1, 3, 3, 5, 3, 9, 3, 7, 5, 9,
    3, 15, 3, 9, 9, 9, 3, 15, 3, 15, 9, 9, 3, 21, 5, 9, 7, 15, 3,
    27, 3, 11, 9, 9, 9, 25, 3, 9, 9, 21, 3, 27, 3, 15, 15, 9, 3, 27,
    5, 15}, for a total of 470, which means that 2080 of the total
    8080200 such equations have a double root, 0.0025…%.

  2. August 7, 2006 pm31 10:47 pm 10:47 pm

    Sorry JBL, the spam filters grabbed your comment, and I only just found it today.

    Thanks for the reasoning. Of course the |100| requirement was arbitrary. If we let 100 get big, the contribution from your first case gets small, and it looks like the percentage goes to 0…

    Jonathan

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