PUMaC 2007 · 组合（A 组） · 第 8 题

PUMaC 2007 — Combinatorics (Division A) — Problem 8

How many pairs of 2007-digit numbers a a · · · a and b b · · · b are there such that a b + 1 2 2007 1 2 2007 1 1 c f a b + · · · a b is even? Express your answer as a ∗ b + d ∗ e for integers a , b , c , d , e , and f 2 2 2007 2007 with a 6 | b and d 6 | e .

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