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A239908
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a(n) = smallest value of k such that the concatenation 1,n,n^2,..., n^(k-1),n^k,n^(k-1),...n^2,n,1 is a prime.
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1
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OFFSET
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1,1
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COMMENTS
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LINKS
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EXAMPLE
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a(1) = 9 as 1 (19 times) = (10^19-1)/9 is the smallest prime of this form with an odd number of 1's.
a(2) = 2 as 12421 is a prime.
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MATHEMATICA
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Do[k = 0; While[a = Table[n^i, {i, 0, k}]; b = ToExpression[ StringJoin[ ToString /@ Join[a, Drop[ Reverse[a], 1]]]]; !PrimeQ[b], k++ ]; Print[k], {n, 1, 10} ] (* Robert G. Wilson v *)
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CROSSREFS
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KEYWORD
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more,nonn,base
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AUTHOR
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STATUS
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approved
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