A068683 Primes formed by the concatenation p,q,p where p and q are successive primes and p>q.

757, 131113, 171317, 231923, 312931, 474347, 978997, 10197101, 107103107, 127113127, 139137139, 151149151, 167163167, 367359367, 421419421, 433431433, 461457461, 617613617, 643641643, 701691701, 863859863, 911907911, 991983991, 102110191021, 103110211031, 103310311033, 104910391049

Offset

1,1

Links

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

Example

171317 is a prime which is the concatenation of 17, 13 and 17.

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:
R:= NULL: count:= 0: p:= 2:
while count < 100 do
  q:= p; p:= nextprime(p);
  v:= cat3(p, q, p);
  if isprime(v) then R:= R, v; count:= count+1;
  fi
od:
R; # Robert Israel, Jul 01 2025

Mathematica

Select[FromDigits[Flatten[IntegerDigits/@#]]&/@({Last[#], First[#], Last[#]} &/@Partition[Prime[Range[200]], 2, 1]), PrimeQ] (* Harvey P. Dale, Oct 25 2011 *)

Crossrefs

Sequence in context: A173507 A101833 A261858 * A221340 A290738 A104336

Adjacent sequences: A068680 A068681 A068682 * A068684 A068685 A068686

Keyword

base,nonn

Author

Amarnath Murthy, Mar 02 2002

Extensions

More terms from Sascha Kurz, Mar 26 2002

More terms from Robert Israel, Jul 02 2025

Status

approved