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# My Problem Series No.1

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Hi everybody! This is the first set of 5 problem sets of problems I will be giving out over a week.

P1 - Easy Combinatorics

Given $20$ apples and $5$ people, how many ways are there to distribute the apples among the people so that each person gets at least one?

P2 - Medium Number Theory

Show that $n^7-n$ is divisible by $42$ for every positive integer $n$.

Source: Putnam Number Theory

P3 - Very Hard Functional Equation

Find all functions $f:\mathbb{R}^+\rightarrow\mathbb{R}^+$ such that for any positive real numbers $x$ and $y$,

$$f(x+f(x)+f(y))=x+f(x+y)$$
Proposed by Athanasios Kontogeorgis, Grecce, and Dorlir Ahmeti, Kosovo

Source: FEOO Shortlist A5

Have fun solving them! If you are stuck, just message me and I'll send a hint.

Feb 13, 2021