This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A068646 #20 Jan 17 2019 13:44:05
%S A068646 11,151,15551,155555555555555555551,155555555555555555555555555555551
%N A068646 Primes in which a string of 5's is sandwiched between two 1's.
%C A068646 The next term is too big to include. - _Sascha Kurz_, Mar 19 2002
%H A068646 Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp151">PDP Reference Table - 151</a>.
%H A068646 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/15551.htm#prime">Prime numbers of the form 155...551</a>.
%H A068646 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e A068646 11 is also a member in which a string of 0 5's is there between two one's.
%p A068646 a := 1:b := 5:i := 1:for n from 0 to 500 do c := a+10*(10^n-1)/9*b+10^(n+1)*a; if(isprime(c)) then d[i] := c; i := i+1; end if; end do:q := seq(d[j],j=1..i-1);
%t A068646 Select[Table[FromDigits[Join[PadRight[{1},n,5],{1}]],{n,500}],PrimeQ] (* _Harvey P. Dale_, Aug 08 2012 *)
%Y A068646 Cf. A068645-A068651, A056246, A082699.
%K A068646 base,nonn
%O A068646 1,1
%A A068646 _Amarnath Murthy_, Feb 28 2002
%E A068646 More terms from _Sascha Kurz_, Mar 19 2002
%E A068646 Edited by _Ray Chandler_, Nov 04 2014