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

Author

Amarnath Murthy, Mar 02 2002

Keywords

• base
• nonn

Examples

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

Links

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

Programs

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 *)

Extensions

More terms from Sascha Kurz, Mar 26 2002

More terms from Robert Israel, Jul 02 2025