Team Selection Test 2010 #0j3b

Team Selection Test 2010Hard

From module Generating Functions (Intro)

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Understand the scale

Scale 0.5: Easiest math competition problems, often solvable even for early middle school students without experience. 1: Problems strictly for beginners, often at a moderate middle school level. 1.5: Problems for stronger beginner students, on the level of the middling problems in most middle school contests. 2: For motivated beginners, harder questions from the previous categories. 2.5: More advanced beginner problems, hardest questions from previous categories. 3: Early intermediate problems that require more creative thinking. 3.5: Tougher early intermediate problems that consistently stretch into higher-level creative thinking or conceptual knowledge. 4: Intermediate-level problems. 4.5: Upper intermediate problems approaching the upper bound for non-invitational sprint math competitions. 5: More difficult AIME problems or simple proof-based Olympiad-style problems. 6: High-level AIME-styled questions or introductory Olympiad-level questions. 7: Tougher Olympiad-level questions, may require more technical knowledge. 8: High-level Olympiad-level questions. 9: Expert Olympiad-level questions. 9.5: The hardest problems appearing on Olympiads which the strongest students could reasonably solve. 10: Historically notorious problems, generally unsuitable for even very hard competitions because they are exceedingly tedious, long, and difficult.

Counting two waysInclusion-exclusionGenerating functionsInduction / smoothing

Problem

Let $m$ , $n$ be positive integers with $m \ge n$ , and let $S$ be the set of all ordered $n$ -tuples $(a_1, a_2, \dots, a_n)$ of positive integers such that $a_1 + a_2 + \dots + a_n = m$ . Show that

\sum_{S} 1^{a_1} 2^{a_2} \cdots n^{a_n} = \binom{n}{n} n^m - \binom{n}{n-1} (n-1)^m + \cdots + (-1)^{n-2} \binom{n}{2} 2^m + \cdots + (-1)^{n-1} \binom{n}{1}.

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