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 A068650 #19 Dec 18 2022 12:25:07
%S A068650 313,3111111111113,311111111111113,3111111111111111111111111111113,
%T A068650 311111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111113
%N A068650 Primes in which a string of 1's is sandwiched between two 3's.
%C A068650 The next term (a(6)) has 127 digits. - _Harvey P. Dale_, Dec 18 2022
%H A068650 Harvey P. Dale, <a href="/A068650/b068650.txt">Table of n, a(n) for n = 1..8</a>
%H A068650 Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp313">PDP Reference Table - 313</a>.
%H A068650 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/31113.htm#prime">Prime numbers of the form 311...113</a>.
%H A068650 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%p A068650 a := 3:b := 1: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 A068650 Select[Table[10*FromDigits[PadRight[{3},n,1]]+3,{n,2,110}],PrimeQ] (* _Harvey P. Dale_, Dec 18 2022 *)
%Y A068650 Cf. A068645-A068651, A056251, A082704.
%K A068650 base,nonn
%O A068650 1,1
%A A068650 _Amarnath Murthy_, Feb 28 2002
%E A068650 More terms from _Sascha Kurz_, Mar 19 2002
%E A068650 Edited by _Ray Chandler_, Nov 04 2014