The numbers from $1$ to $2017$ are written on a board. Deka and Farid play the following game : each of them, on his turn, erases one of the numbers. Anyone who erases a multiple of $2, 3$ or $5$ loses and the game is over. Is there a winning strategy for Deka ?
Problem
Source: PAMO 2017 Problem 5
Tags: combinatorics, game strategy
05.07.2017 23:04
An equivalent problem: For $ \mathbb{N} $ denoting the non-negative numbers, find the parity of the set $$ \left\{ (k,l,m)\in\mathbb{N}^3\setminus\{ (0,0,0)\}|2^k3^l5^m\le 2017\right\} $$
06.07.2017 17:12
Let $A_1,A_2$ and $A_3$ denote the set of numbers less than or equal to $2017$ that are divisible by $2,3$ and $5$ respectively. Note that \begin{align*} \left| A_1\cup A_2\cup A_3\right| &=|A_1|+|A_2|+|A_3|-|A_1\cap A_2|-|A_2\cap A_3|-|A_3\cap A_1|+|A_1\cap A_2\cap A_3| \\ &=1008+672+403-336-134-201+67 \\ &=1479. \end{align*} Hence after $2017-1479=538$ turns, the player is destined to lose. Thus the person starting first loses assuming that both the players play optimally.
30.07.2017 20:51
It was pointed out to me by one of the South African students that the question only asks if a winning strategy exists, and so, in principle, "The game clearly ends in a finite number of turns, and can not end in a draw, and both players have perfect information etc..., and so there is a winning strategy for either the first or the second player. Deka's winning strategy is to play as the player with a winning strategy and to follow that winning strategy." is a perfectly correct solution.
02.12.2017 18:32
Lol, unless the jury feels sympathetic or humorously love his/her answer, I am sure the rubric asks to show "some" work for the full score aha
15.08.2024 00:00
Let's use Euler's Totient function to find the number co-prime to 2,3,5 and less than 2017. The larget number that is a multiple of 2, 3, and 5 and less than 2017 is $\boxed{2010}$. Using this fact the prime factorization of 2010 is: $$2010 = 2\cdot 3 \cdot 5 \cdot 67$$ By using the previous preposition, we can get the number of the Co-prime numbers less than 2010 which is what we need to progress in the problem. $$\phi(2010) = 2010 \left(1-\frac{1}{2}\right) \left(1-\frac{1}{3}\right)\left(1-\frac{1}{5}\right) \left(1-\frac{1}{67}\right) = \boxed{528}$$ Then we know that for sure there are 528 numbers that are not multiples of 2, 3, and 5. However, we need to include the Co-prime number that are $$2010 < n < 2017$$and the numbers that are a multiple of 67 but not a multiple of 2, 3, and 5. These numbers are: $$S = \{67, 469, 737, 871, 1139, 1273, 1541, 1943, 2011, 2017 \}$$$$|S| = 10$$ Therefore, the final answer is: $$528 + 10 = \boxed{538}$$.
15.08.2024 00:01
Mazen_hisham123 wrote: Let's use Euler's Totient function to find the number co-prime to 2,3,5 and less than 2017. The larget number that is a multiple of 2, 3, and 5 and less than 2017 is $\boxed{2010}$. Using this fact the prime factorization of 2010 is: $$2010 = 2\cdot 3 \cdot 5 \cdot 67$$ By using the previous preposition, we can get the number of the Co-prime numbers less than 2010 which is what we need to progress in the problem. $$\phi(n) = 2010 \left(1-\frac{1}{2}\right) \left(1-\frac{1}{3}\right)\left(1-\frac{1}{5}\right) \left(1-\frac{1}{67}\right) = \boxed{528}$$ Then we know that for sure there are 528 numbers that are not multiples of 2, 3, and 5. However, we need to include the Co-prime number that are $$2010 < n < 2017$$and the numbers that are a multiple of 67 but not a multiple of 2, 3, and 5. These numbers are: $$S = \{67, 469, 737, 871, 1139, 1273, 1541, 1943, 2011, 2017 \}$$$$|S| = 10$$ Therefore, the final answer is: $$528 + 10 = \boxed{538}$$. Brilliant move