The numbers $1,2,3,\dots,2017$ are on the blackboard. Amelie and Boris take turns removing one of those until only two numbers remain on the board. Amelie starts. If the sum of the last two numbers is divisible by $8$, then Amelie wins. Else Boris wins. Who can force a victory?
Problem
Source: Bundeswettbewerb Mathematik 2017, Round 1 - #1
Tags: blackboard, combinatorics, combinatorics unsolved, number theory, number theory unsolved, divisible, Divisibility
08.08.2017 01:27
08.08.2017 01:35
Sniped
20.08.2017 17:45
Wouldn't this be more simplistic hmmmm: $A$ chooses $2017$. Whatever $B$ chooses, say $x$, $A$ chooses $2016-x$. If $B$ chooses $1008$ or $2016$, $A$ chooses the other one of the two. Then the two numbers left will sum up to $2016$ or $3024$ (that is, if the two numbers left are $1008$ and $2016$).
23.01.2018 22:40
Say he will chose $x$ then there will always remain on board such that she can choose $8k–x$ , $8|a+b$ as they take turns there will always remain an $8k$ and b chose $2016,1008$she will be clever enough and will choose either of them to generate an $a+b=8k$ $k=Z$