A056803 Numbers k such that k copies of 12 followed by 1 is a palindromic prime.

3, 5, 21, 69, 313, 699, 798, 989, 3904, 7029, 23249

Offset

1,1

Links

Table of n, a(n) for n=1..11.

P. De Geest, SUPP Reference Table

Example

12121212121 is prime so 5 is a term.

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

Definition clarified by N. J. A. Sloane, Nov 09 2024

a(11) from Michael S. Branicky, Dec 11 2024

Status

approved