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A372262
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a(n) = smallest number m > 0 such that n followed by m 3's yields a prime, or -1 if no such m exists.
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3
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1, 1, -1, 1, 1, -1, 1, 1, -1, 1, 1, -1, 14, 2, -1, 1, 1, -1, 1, 3, -1, 1, 1, -1, 8, 1, -1, 1, 1, -1, 1, 4, -1, 2, 1, -1, 1, 1, -1, 483, 2, -1, 1, 1, -1, 1, 2, -1, 2, 1, -1, 1, 2, -1, 3, 1, -1, 6, 1, -1, 1, 5, -1, 1, 1, -1, 1, 1, -1, 5, 3, -1, 1, 1, -1, 3, 1, -1, 2, 4
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OFFSET
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1,13
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COMMENTS
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a(n) = -1 when n = 3*k because no matter how many 3's are appended to n, the resulting number is always divisible by 3 and therefore cannot be prime.
a(n) = -1 when n = 37037*k + 2808, 3666, 4070, 9287, 18799, 21574, 28083, 30558, 33300, 33740, 36663 or 36707, because n followed by any positive number, m say, of 3's is divisible by at least one of the primes {7,11,13,37}.
a(817) > 300000 or a(817) = -1.
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LINKS
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EXAMPLE
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a(20)=3 because 203 and 2033 are composite but 20333 is prime.
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CROSSREFS
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KEYWORD
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sign,base
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AUTHOR
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STATUS
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approved
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