A056803 Numbers n such that 12 repeated n times followed by 1 is a palindromic prime.

3, 5, 21, 69, 313, 699, 798, 989, 3904, 7029

Offset

1,1

Links

Table of n, a(n) for n=1..10.

P. De Geest, SUPP Reference Table

Example

12121212121 is prime so 5 appears in the sequence.

Mathematica

Do[m = n; If[PrimeQ[120(10^(2n) - 1)/99 + 1], Print[n]], {n, 1, 600}]
(IntegerLength[#]-1)/2&/@Select[10#+1&/@Table[FromDigits[Flatten[ IntegerDigits/@ PadRight[{}, n, {1, 2}]]], {n, 2, 15000, 2}], PrimeQ] (* Harvey P. Dale, Apr 02 2020 *)

Crossrefs

Corresponding primes are given in A092696. Corresponding decimal digit lengths are given in A062209. a(k) = (A062209(k-1)-1)/2.

Sequence in context: A266203 A264683 A286033 * A071706 A196418 A189560

Adjacent sequences: A056800 A056801 A056802 * A056804 A056805 A056806

Keyword

nonn,base

Author

Robert G. Wilson v, Aug 22 2000

Extensions

More terms from Rick L. Shepherd, Mar 04 2004

Status

approved