A090265 Numbers n such that the strings n9n and 9n9 are both prime.

1, 19, 41, 43, 47, 61, 67, 71, 73, 83, 107, 149, 211, 217, 221, 269, 277, 319, 437, 443, 523, 547, 553, 607, 619, 647, 679, 697, 817, 863, 877, 899, 1171, 1283, 1297, 1379, 1481, 1547, 1621, 1853, 1867, 1967, 1993, 1997, 2021, 2053, 2107, 2357, 2359, 2393

Offset: 1

Author

Amarnath Murthy, Nov 28 2003

Keywords

• base
• nonn

Examples

			41 is a member as 41941 and 9419 are both prime.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A090262, A090263, A090264.

Programs

Mathematica

n9Q[n_]:=Module[{idn=IntegerDigits[n]},AllTrue[{FromDigits[Join[idn,{9}, idn]],FromDigits[ Join[{9},idn,{9}]]},PrimeQ]]; Select[Range[2500],n9Q] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Feb 01 2016 *)

Extensions

More terms from mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 02 2004