%I
%S 12,13,15,19,27,31,34,36,40,42,45,49,57,58,61,69,70,72,78,82,87,90,91,
%T 96,97,1000,1002,1017,1018,1024,1033,1035,1063,1068,1069,1074,1084,
%U 1086,1090,1095,1110,1114,1116,1117,1126,1128,1173,1174,1179,1185,1189,1192
%N Values of n for which the concatenation of the form 1nn1 (sequence A100846) are primes.
%C All terms == 0 or 1 (mod 3), and have an even number of decimal digits.  _Robert Israel_, May 09 2017
%H Robert Israel, <a href="/A102496/b102496.txt">Table of n, a(n) for n = 1..10000</a>
%e For n=12 we have 112121, which is prime.
%e For n=13 we have 113131, which is prime.
%e For n=1000 we have 1100010001, which is prime.
%p f:= n > 1 + 1*10^(2*ilog10(n)+3)+(n)*(10+10^(2+ilog10(n))):
%p select(n > isprime(f(n)), [$1..2000]); # _Robert Israel_, May 09 2017
%t Select[Range@ 1200, PrimeQ[FromDigits@ Join[{1}, #, #, {1}]] &@ IntegerDigits[#] &] (* _Michael De Vlieger_, May 09 2017 *)
%Y Cf. A100846. The primes themselves are in sequence A102497.
%K easy,nonn,base
%O 1,1
%A Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 12 2005
