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Problem 5, Olympic Revenge 2010

Source: IX Olympic Revenge - 2010

January 28, 2013
combinatorics proposedcombinatorics

Problem Statement

Secco and Ramon are drunk in the real line over the integer points aa and bb, respectively. Our real line is a little bit special, though: the interval (,0)(-\infty, 0) is covered by a sea of lava. Being aware of this fact, and also because they are drunk, they decided to play the following game: initially they choose an integer number k>1k>1 using a fair dice as large as desired, and therefore they start the game. In the first round, each player writes the point hh for which it wants to go.
After that, they throw a coin: if the result is heads, they go to the desired points; otherwise, they go to the points 2gh2g - h, where gg is the point where each of the players were in the precedent round (that is, in the first round g=ag = a for Secco and g=bg = b for Ramon). They repeat this procedure in the other rounds, and the game finishes when some of the player is over a point exactly kk times bigger than the other (if both of the player end up in the point 00, the game finishes as well).
Determine, in values of kk, the initial values aa and bb such that Secco and Ramon has a winning strategy to finish the game alive.
Observation: If any of the players fall in the lave, he dies and both of them lose the game