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 A068673 #19 Aug 21 2024 12:26:39
%S A068673 1,3,7,13,27,31,49,57,63,81,91,97,99,103,109,117,123,129,151,153,181,
%T A068673 187,193,201,213,231,237,259,279,297,301,319,327,367,409,423,427,439,
%U A068673 459,483,487,493,523,543,553,559,571,579,601,609,613,621,627,657,669
%N A068673 Numbers which yield primes when a 1 is prefixed or appended.
%e A068673 57 is a term as 157 as well as 571 are primes.
%p A068673 i := 0:for k from 1 to 3200 do if(isprime(1+10*k) and isprime(k+10^(ceil(evalf(log(k+1)/log(10))))*1)) then s[i] := k; i := i+1; end if; end do:q := seq(s[j],j=0..i-1);
%t A068673 p1Q[n_]:=Module[{idn=IntegerDigits[n]},PrimeQ[FromDigits[Join[{1}, idn]]] && PrimeQ[FromDigits[Join[idn,{1}]]]]; Select[Range[700],p1Q] (* _Harvey P. Dale_, Jan 23 2013 *)
%t A068673 Flatten@Position[{#+10^IntegerLength@#,1+10*#}&/@Range@10000,{_?PrimeQ..}] (* _Hans Rudolf Widmer_, Aug 21 2024 *)
%K A068673 base,easy,nonn
%O A068673 1,2
%A A068673 _Amarnath Murthy_, Mar 02 2002
%E A068673 More terms from _Sascha Kurz_, Mar 17 2002