A239908 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.

9, 2, 1, 12, 1, 2

Offset

1,1

Comments

Terms from Robert G. Wilson v.

Links

Table of n, a(n) for n=1..6.

Example

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.

Mathematica

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 *)

Crossrefs

See A069566 for another version.

Sequence in context: A197757 A293258 A010536 * A293171 A334689 A388928

Adjacent sequences: A239905 A239906 A239907 * A239909 A239910 A239911

Keyword

more,nonn,base

Author

Ray Chandler and N. J. A. Sloane, Apr 08 2014

Status

approved