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

Author

Robert G. Wilson v, Aug 22 2000

Keywords

• nonn
• base

Examples

			12121212121 is prime so 5 is a term.
		

Links

• P. De Geest, SUPP Reference Table

Crossrefs

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

Programs

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 *)

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