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Math Help - Using generating functions with recursion?

  1. #1
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    Using generating functions with recursion?

    Let a0 = 0, a1 = 2, and a2 = 5. Use generating functions to solve the recurrence
    equation an+3 = 5an+2 −7an+1 +3an +2n for n ≥ 0.

    I first got:
    f(x) = a0 + a1x + a2x^2 +.... + anx^n + ...
    -5xf(x) = 0 - 5a0x - 5a1x^2 - ... - 5a(n-1)x^n
    7x^2f(x) = ... + 7an-2^n - ...
    -3x^3 f(x) = ... - 3an-3x^n - ...

    i got it down to 1-5x+7x^2 - 3x^3 = (2x-4x^2 + 8x^3)/1-2x

    Please help me out, I have to do a couple of these and If i understand one of them, I will understand all of them.
    Thanks!
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  2. #2
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    Re: Using generating functions with recursion?

    Hey haytham12121.

    If your equation involving the x's is correct, then you will need to find the roots for the equation which will look like a quartic.

    There is a complicated formula to solve quartics, but you can use something known as the rational root theorem to check for rational roots as well as the option of using a computer to calculate the roots straight away.

    To get the polynomial, multiply both sides by (1-2x), collect terms and simplify to get a polynomial P(x) where P(x) = 0 and consider the above to get those roots.
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