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A112386
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Smallest prime obtained by appending one or more 1's to n, -1 if no such prime exists.
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6
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11, 211, 31, 41, 511111, 61, 71, 811, 911, 101, 1111111111111111111, 121111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111, 131, 14111111111, 151, 16111, 1711111111, 181, 191, 2011, 211, 22111, 2311, 241
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OFFSET
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1,1
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COMMENTS
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a(37) = -1 since there is a covering of the set {371, 3711, 37111, ...} by the prime moduli 3, 7, 13, 37. Hence, there are infinitely many values -1 in the sequence (at 371, 3711, 37111, ...). - Emmanuel Vantieghem, Oct 27 2022
a(38) = -1 because 38 followed by m >= 1 1's is divisible by 3 or 37 or by (7*10^k-1)/3 if m = 3k. - Toshitaka Suzuki, Nov 07 2023
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LINKS
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EXAMPLE
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a(5) = 511111 because 51, 511, 5111 and 51111 are not primes.
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MATHEMATICA
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f[n_] := Block[{k = 1, e = Floor[Log[10, n] + 1]}, While[ !PrimeQ[n*10^k + (10^k - 1)/9], k++ ]; n*10^k + (10^k - 1)/9]; Array[f, 24] (* Robert G. Wilson v, Dec 05 2005 *)
Table[SelectFirst[Table[FromDigits[PadRight[IntegerDigits[k], n, 1]], {n, IntegerLength[k]+1, 250}], PrimeQ], {k, 25}] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Nov 30 2017 *)
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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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Michel Dauchez (mdzdm(AT)yahoo.fr), Dec 04 2005
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EXTENSIONS
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STATUS
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approved
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