A034814 Concatenations C1 and C2 are both prime (see the comment lines).

9, 51, 63, 87, 111, 123, 153, 177, 207, 211, 239, 263, 273, 289, 327, 333, 343, 359, 407, 471, 489, 497, 513, 541, 597, 621, 651, 659, 663, 681, 687, 693, 697, 747, 753, 793, 819, 831, 869, 909, 977, 987, 1027, 1041, 1089, 1131, 1143, 1239, 1491, 1611

Offset

1,1

Comments

C1 = 'prevprime(k) followed by k'.

C2 = 'k followed by prevprime(k)'.

Links

Andrew Howroyd, Table of n, a(n) for n = 1..1000

Example

n=747 -> previous prime is 743, thus '743747' and '747743' are both primes.

Mathematica

c1c2Q[n_]:=With[{pp=NextPrime[n, -1]}, AllTrue[{n*10^IntegerLength[pp]+pp, pp*10^IntegerLength[n]+n}, PrimeQ]]; Select[Range[2000], c1c2Q] (* Harvey P. Dale, Jun 20 2026 *)

Crossrefs

Intersection of A034808 and A034809.

Cf. A034810-A034821.

Sequence in context: A221246 A002462 A210061 * A034816 A140381 A330805

Adjacent sequences: A034811 A034812 A034813 * A034815 A034816 A034817

Keyword

nonn,base,changed

Author

Patrick De Geest, Oct 15 1998

Extensions

Offset changed by Andrew Howroyd, Aug 13 2024

Status

approved