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Saudi Arabian Mathematical Competitions, 2012

Algebra Difficulty 5.8 AIME, harder Prove it Saudi Arabia

Determine if there are polynomials p(x)p(x) and q(x)q(x) with real coefficients such that
p(n)q(n)=1+12!+13!+⋯+1n! \frac{p(n)}{q(n)} = 1 + \frac{1}{2!} + \frac{1}{3!} + \dots + \frac{1}{n!}
for every positive integer nn.

Solution

Assume that there are polynomials p,q∈R[X]p, q \in \mathbb{R}[X] such that
p(n)q(n)=1+12!+⋯+1n!,n≥1.(1) \frac{p(n)}{q(n)} = 1 + \frac{1}{2!} + \dots + \frac{1}{n!}, \quad n \ge 1. \quad (1)
Then
p(n+1)q(n+1)−p(n)q(n)=1(n+1)!,n≥1, \frac{p(n+1)}{q(n+1)} - \frac{p(n)}{q(n)} = \frac{1}{(n+1)!}, \quad n \ge 1,
so
p(n+1)q(n)−p(n)q(n+1)q(n)q(n+1)=1(n+1)!,n≥1.(2) \frac{p(n+1)q(n) - p(n)q(n+1)}{q(n)q(n+1)} = \frac{1}{(n+1)!}, \quad n \ge 1. \quad (2)
Define the polynomials u,v∈R[X]u, v \in \mathbb{R}[X] by
u(x)=p(x+1)q(x)−p(x)q(x+1),v(x)=q(x)q(x+1). u(x) = p(x+1)q(x) - p(x)q(x+1), \quad v(x) = q(x)q(x+1).
From (2) it follows that uu is not the 0-constant polynomial and we have
u(n)v(n)=1(n+1)!,n≥1(3) \frac{u(n)}{v(n)} = \frac{1}{(n+1)!}, \quad n \ge 1 \quad (3)
and
u(n+1)v(n+1)=1(n+2)!,n≥1. \frac{u(n+1)}{v(n+1)} = \frac{1}{(n+2)!}, \quad n \ge 1.
It follows
u(n+1)u(n)⋅v(n)v(n+1)=1n+2,n≥1.(4) \frac{u(n+1)}{u(n)} \cdot \frac{v(n)}{v(n+1)} = \frac{1}{n+2}, \quad n \ge 1. \quad (4)
We have
lim⁡n→∞u(n+1)u(n)=lim⁡n→∞v(n+1)v(n)=1, \lim_{n \to \infty} \frac{u(n+1)}{u(n)} = \lim_{n \to \infty} \frac{v(n+1)}{v(n)} = 1,
and from (4) we obtain the contradiction 1=01 = 0.

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