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 A068674 #13 May 25 2019 22:10:26
%S A068674 1,7,11,17,31,37,59,67,73,109,119,121,137,169,187,191,221,229,259,271,
%T A068674 331,343,359,361,373,407,413,449,467,499,511,527,533,539,541,557,581,
%U A068674 607,613,617,637,673,701,719,733,739,767,779,793,823,851,889,917,929
%N A068674 Numbers which yield primes when a 3 is prefixed or appended.
%e A068674 73 is a term as both 373 and 733 are primes.
%p A068674 i := 0:for k from 1 to 3200 do if(isprime(3+10*k) and isprime(k+10^(ceil(evalf(log(k+1)/log(10))))*3)) then s[i] := k; i := i+1; end if; end do:q := seq(s[j],j=0..i-1);
%t A068674 Select[Range[930],Union[PrimeQ[FromDigits/@{Append[(x=IntegerDigits[#]),3],Prepend[x,3]}]]=={True} &] (* _Jayanta Basu_, May 20 2013 *)
%Y A068674 Cf. A068673.
%K A068674 base,easy,nonn
%O A068674 1,2
%A A068674 _Amarnath Murthy_, Mar 02 2002
%E A068674 More terms from _Sascha Kurz_, Mar 17 2002