$\textbf{Problem C.1}$ There are two piles of coins, each containing $2010$ pieces. Two players $A$ and $B$ play a game taking turns ($A$ plays first). At each turn, the player on play has to take one or more coins from one pile or exactly one coin from each pile. Whoever takes the last coin is the winner. Which player will win if they both play in the best possible way?
Problem
Source: JBMO Shortlist 2010 Problem C1 (Combinatorics #1)
Tags: combinatorics unsolved, combinatorics
25.01.2015 17:04
This is also here. Solution is provided here.
30.09.2020 11:47
B takes coins as well A. HAHAHAHA ITS EASY QUESTION
21.11.2023 18:16
@Above I think that's not true. If $B$ does the same as $A$, $B$ can loose. Example: $A:(2010,2010)\rightarrow (1,2010)$ $B:(1,2010)\rightarrow (1,1)$ $A:(1,1)\rightarrow (0,0)$ and $A$ wins.
21.11.2023 18:27
$B$ wins. Claim: If one of them gets $(1,2)$, then he wins. Proof:$(1,2)\rightarrow (1,1),(0,1),(0,2)\rightarrow (0,0)$ and the person who wrote $(1,2)$ wins. Claim: If one of them gets $(3,3)$, then he wins. Proof:$(3,3)\rightarrow (0,3)\rightarrow (0,0)$ and $(3,3)\rightarrow (1,3),(2,3),(2,2)\rightarrow (1,2)$ and we know that the person who writes $(1,2)$ wins. Let's prove that the person who writes $(3k,3k)$ and $(3k+1,3k+2)$ wins by induction. Let it be true for $k=1,2,...,m$ and let's prove that it's also true for $m+1$. Let $X$ write $(3m+3,3m+3)$. If $Y:(3m+3,3m+3)\rightarrow (3n,3m+3)$, then $X:(3n,3m+3)\rightarrow (3n,3n)$ and wins. If $Y:(3m+3,3m+3)\rightarrow (3n+1,3m+3)$, then $X:(3n+1,3m+3)\rightarrow (3n+1,3n+2)$ and wins. If $Y:(3m+3,3m+3)\rightarrow (3n+2,3m+3)$, then $X:(3n+2,3m+3)\rightarrow (3n+2,3n+1)$ and wins. If $Y:(3m+3,3m+3)\rightarrow (3m+2,3m+2)$, then $X:(3m+2,3m+2)\rightarrow (3m+1,3m+2)$ and wins. Let $X$ write $(3m+4,3m+5)$. If $Y:(3m+4,3m+5)\rightarrow (3n,3m+\{4,5\})$, then $X:(3n,3m+\{4,5\})\rightarrow (3n,3n)$ and wins. If $Y:(3m+4,3m+5)\rightarrow (3n+1,3m+\{4,5\})$ and $n<m+1$, then $X:(3n+1,3m+\{4,5\})\rightarrow (3n+1,3n+2)$ and wins. If $Y:(3m+4,3m+5)\rightarrow (3m+4,3m+4)$, then $X:(3m+4,3m+4)\rightarrow (3m+3,3m+3)$ and wins. If $Y:(3m+4,3m+5)\rightarrow (3n+2,3m+\{4,5\})$, then $X:(3n+2,3m+\{4,5\})\rightarrow (3n+2,3n+1)$ and wins. If $Y:(3m+4,3m+5)\rightarrow (3m+3,3m+4)$, then $X:(3m+3,3m+4)\rightarrow (3m+3,3m+3)$ and wins. Since $3|2010$, $B$ wins.