A103967 - OEIS

%I #21 Sep 08 2022 08:45:17

%S 3,9,17,21,27,29,39,41,47,51,59,71,81,87,89,113,117,119,131,137,141,

%T 159,161,177,189,201,239,263,287,291,299,333,359,371,377,389,393,399,

%U 443,459,461,467,471,489,509,539,573,603,623,653,657,659,663,671,681

%N Numbers n such that the string n1n is the decimal expansion of a prime number.

%C All terms are odd and not multiples of 5. - _Zak Seidov_, Oct 29 2014

%H Zak Seidov, <a href="/A103967/b103967.txt">Table of n, a(n) for n = 1..10000</a>

%e n=3 is in the sequence because n1n = 313 is prime.

%e n=41 is in the sequence because n1n = 41141 is prime.

%e n=89 is in the sequence because n1n = 89189 is prime.

%t Reap[Do[If[Mod[n,5]>0&&PrimeQ[ToExpression[ToString[n]<>"1"<>ToString[n]]], Sow[n]],{n,3,1000,2}]][[2,1]] (* _Zak Seidov_, Oct 29 2014 *)

%o (Magma) [ n: n in [0..800] | IsPrime(Seqint(Intseq(n) cat [1] cat Intseq(n))) ]; // _Klaus Brockhaus_, Feb 03 2011

%Y Cf. A249374.

%K base,nonn

%O 1,1

%A _Parthasarathy Nambi_, Mar 30 2005