olymmath

IT-CESENATICO 2010 Problem 5

    In the land of Cockaigne, people play the following solitaire. It starts from a finite string of zeros and ones, and are granted the following moves: (i) cancel each two consecutive ones; (ii) delete three consecutive zeros; (iii) if the substring within the string is 0101 , one may replace this by substring 100100 . The moves (i), (ii) and (iii) must be made one at a time. You win if you can reduce the string to a string formed by two digits or less. (For example, starting from 01010101 , one can win using move (iii) first in the last two digits, resulting in 0110001100 , then playing the move (i) on two 'ones', and finally the move (ii) on the three zeros, one will get the empty string.) Among all the 10241024 possible strings of ten-digit binary numbers, how many are there from which it is not possible to win the solitary?

    Dataset text AI-MO AoPS. Quoted with source.

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