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 A068649 #21 Jan 17 2019 13:44:05
%S A068649 11,191,19991,199999991,19999999999999999999999999999999999999991,
%T A068649 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991
%N A068649 Primes in which a string of 9's is sandwiched between two 1's.
%C A068649 The next term has 199 9's sandwiched between the starting and ending 1.
%H A068649 Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp191">PDP Reference Table - 191</a>.
%H A068649 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/19991.htm#prime">Prime numbers of the form 199...991</a>.
%H A068649 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e A068649 11 is also a member in which a string of 0 9's is there between two one's.
%p A068649 a := 1:b := 9: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 A068649 Select[Table[FromDigits[Join[{1}, Table[9, {i}], {1}]], {i, 0, 200}], PrimeQ]
%Y A068649 Cf. A056250, A068645, A068646, A068647, A068648, A068650, A068651, A082703.
%K A068649 base,nonn
%O A068649 1,1
%A A068649 _Amarnath Murthy_, Feb 28 2002
%E A068649 More terms from _Sascha Kurz_ and _Harvey P. Dale_, Mar 19 2002
%E A068649 Edited by _Ray Chandler_, Nov 04 2014