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A345698
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Sierpiński problem in base 5: a(n) is the smallest k >= 0 such that (2*n)*5^k + 1 is prime, or -1 if no such k exists.
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0
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0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 2, 1, 0, 0, 3, 8, 0, 1, 0, 0, 3, 0, 1, 1, 0, 1, 1, 0, 0, 1, 2, 0, 3, 0, 0, 257, 2, 0, 1, 0, 1, 1, 0, 2, 1, 2, 0, 1, 0, 0, 1, 0, 0, 3, 0, 1, 15, 4, 1, 79, 48, 0, 1, 0, 1, 5, 0, 0, 1, 6, 4, 3, 0, 0, 1, 2, 0, 3, 2, 0, 1, 0, 2, 7
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OFFSET
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1,12
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COMMENTS
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a(159986/2) = a(79993) = -1.
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LINKS
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EXAMPLE
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For n = 17: 34*5^k + 1 is composite for k = 0, 1, 2, 3, 4, 5, 6, 7 and prime for k = 8. Since 8 is the smallest such k, a(17) = 8.
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PROG
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(PARI) a(n) = for(k=0, oo, if(ispseudoprime((2*n)*5^k+1), return(k)))
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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