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A227691
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Winning positions in the Subtract-a-Prime game.
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4
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2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30, 31, 32, 33, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74
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OFFSET
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1,1
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COMMENTS
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Consider the following game: two players make moves in turn, initially the number on the board is n. Each move consists of subtracting a prime number that is at most the number on the board. The player who cannot play loses. This sequence is the set of winner positions in this game.
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LINKS
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MATHEMATICA
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moves[n_] := Table[n - Prime[i], {i, 1, PrimePi[n]}]
gana[n_] := gana[n] = If[n < 2, False, ! Select[moves[n], !gana[#] &] == {}];
Select[Range[155], gana[#] &]
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PROG
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(PARI) is(n)=if(isprime(n) || isprime(n-1), return(1)); if(n<15, return(0)); for(k=9, n-1, if(isprime(n-k) && !is(k), return(1))); 0 \\ Charles R Greathouse IV, Nov 13 2013
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CROSSREFS
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The Grundy numbers of this game are in A014589.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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