A068681 Primes formed by the concatenation k, k-2, k.

313, 757, 131113, 171517, 312931, 393739, 474547, 494749, 575557, 939193, 111109111, 127125127, 139137139, 147145147, 151149151, 169167169, 189187189, 213211213, 217215217, 227225227, 237235237, 239237239, 247245247, 253251253, 269267269, 331329331, 333331333, 353351353, 363361363, 377375377, 387385387, 389387389, 393391393

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

111109111 is a term as it is the concatenation of 111, 109 and 111.

Maple

cat3:= proc(a, b, c) local alpha, beta;
    beta:= ilog10(c)+1;
    alpha:= beta + ilog10(b)+1;
    10^alpha*a + 10^beta*b + c
end proc:
select(isprime, [seq(cat3(t, t-2, t), t=3..1000, 2)]); # Robert Israel, Jul 01 2025

Mathematica

Select[Table[FromDigits[Join[#[k], #[k-2], #[k]]], {k, 3, 400, 2}] & [IntegerDigits], PrimeQ] (* Paolo Xausa, Jul 03 2025 *)

Crossrefs

Cf. A068680.

Sequence in context: A104719 A087364 A090514 * A341802 A392227 A082436

Adjacent sequences: A068678 A068679 A068680 * A068682 A068683 A068684

Keyword

base,easy,nonn

Author

Amarnath Murthy, Mar 02 2002

Extensions

More terms from Eli McGowan (ejmcgowa(AT)mail.lakeheadu.ca), Apr 11 2002

Status

approved