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Niels Henrik Abels Math Contest (Norwegian Math Olympiad) Final Round
2023 Abelkonkurransen Finale
2b
2b
Part of
2023 Abelkonkurransen Finale
Problems
(1)
Arne and Berit play a game on a blackboard
Source: 2023 Abelkonkurransen Finale, Problem 2b
3/12/2024
Arne and Berit are playing a game. They have chosen positive integers
m
m
m
and
n
n
n
with
n
≥
4
n\geq 4
n
≥
4
and
m
≤
2
n
+
1
m \leq 2n + 1
m
≤
2
n
+
1
. Arne begins by choosing a number from the set
{
1
,
2
,
…
,
n
}
\{1, 2, \dots , n \}
{
1
,
2
,
…
,
n
}
, and writes it on a blackboard. Then Berit picks another number from the same set, and writes it on the board. They continue alternating turns, always choosing numbers that are not already on the blackboard. When the sum of all the numbers on the board exceeds or equals
m
m
m
, the game is over, and whoever wrote the last number has won. For which combinations of
m
m
m
and
n
n
n
does Arne have a winning strategy?
combinatorics