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A091920
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Smallest n-digit prime with digits from {1,4,7} only, or 0 if no such prime exists.
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1
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7, 11, 0, 1117, 11117, 0, 1111447, 11111117, 0, 1111111411, 11111111741, 0, 1111111111177, 11111111111411, 0, 1111111111111447, 11111111111111171, 0, 1111111111111111111, 11111111111111111447, 0, 1111111111111111111711
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OFFSET
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1,1
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COMMENTS
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Any 3k-digit number containing only digits from {1,4,7} has a digit-sum divisible by 3. Therefore the number is divisible by 3 and a(3k) = 0 for all positive integers k.
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LINKS
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FORMULA
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a(k) <= minimum(A036931(k), A036934(k), A036944(k)), a(3k) = 0 and a(A004023(k)) = (10^A004023(k) - 1)/9 = A004022(k) for all positive integers k. (The inequality above holds iff a(k) contains at least one of each digit 1, 4 and 7.)
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MATHEMATICA
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Flatten[Table[Select[FromDigits/@Tuples[{1, 4, 7}, n], PrimeQ, 1], {n, 25}]/.{}->{0}] (* Jinyuan Wang, Mar 09 2020 *)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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