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Algebra Difficulty 5.3 AIME, harder Find the answer

Let z0+z1+z2+z_{0}+z_{1}+z_{2}+\cdots be an infinite complex geometric series such that z0=1z_{0}=1 and z2013=120132013z_{2013}=\frac{1}{2013^{2013}}. Find the sum of all possible sums of this series.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Clearly, the possible common ratios are the 2013 roots r1,r2,,r2013r_{1}, r_{2}, \ldots, r_{2013} of the equation r2013=120132013r^{2013}=\frac{1}{2013^{2013}}. We want the sum of the values of xn=11rnx_{n}=\frac{1}{1-r_{n}}, so we consider the polynomial whose roots are x1,x2,,x2013x_{1}, x_{2}, \ldots, x_{2013}. It is easy to see that (11xn)2013=120132013\left(1-\frac{1}{x_{n}}\right)^{2013}=\frac{1}{2013^{2013}}, so it follows that the xnx_{n} are the roots of the polynomial equation 120132013x2013(x1)2013=0\frac{1}{2013^{2013}} x^{2013}-(x-1)^{2013}=0. The leading coefficient of this polynomial is 1201320131\frac{1}{2013^{2013}}-1, and it follows easily from the Binomial Theorem that the next coefficient is 2013, so our answer is, by Vieta's Formulae, 20131201320131=20132014201320131-\frac{2013}{\frac{1}{2013^{2013}}-1}=\frac{2013^{2014}}{2013^{2013}-1}

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