A034812 Concatenation of 'prevprime(k) and k and prevprime(k)' is a prime.

5, 8, 9, 12, 14, 17, 19, 23, 25, 28, 31, 33, 38, 39, 41, 42, 47, 48, 51, 60, 61, 62, 63, 69, 71, 72, 75, 77, 78, 80, 81, 84, 85, 91, 102, 104, 105, 111, 124, 126, 149, 150, 167, 181, 182, 189, 192, 194, 215, 222, 227, 230, 233, 242, 243, 256, 271, 273, 283, 288, 308

Offset: 1

Author

Patrick De Geest, Oct 15 1998

Keywords

• nonn
• base

Examples

			n=222 -> previous prime is 211, thus '211222211' is a prime.
		

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A034808-A034821.

Programs

Mathematica

okQ[n_] := Module[{idppn=IntegerDigits[NextPrime[n,-1]]}, PrimeQ[FromDigits[Join[idppn, IntegerDigits[n], idppn]]]]; Select[Range[350], okQ] (* Harvey P. Dale, Jan 07 2011 *)

PARI

isok(n)=my(x=digits(precprime(n-1))); n>2 && isprime(fromdigits(concat([x, digits(n), x]))) \\ Andrew Howroyd, Aug 13 2024

Extensions

Offset changed by Andrew Howroyd, Aug 13 2024