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 A139535 #13 Aug 30 2020 13:50:07
%S A139535 101,1009,10007,100003,1000003,100000037,100000007,1000000007,
%T A139535 100000000019,100000000003,10000000000037,100000000000031,
%U A139535 1000000000000037,10000000000000061,100000000000000013
%N A139535 Smallest prime starting with 1 that contains exactly n 0's.
%H A139535 Harvey P. Dale, <a href="/A139535/b139535.txt">Table of n, a(n) for n = 1..100</a>
%t A139535 sp0[n_]:=Module[{c=1,d},While[d=FromDigits[Join[PadRight[{1},n+1,0],IntegerDigits[ c]]];DigitCount[d,10,0]>n||CompositeQ[d],c=c+2];FromDigits[ Join[ PadRight[ {1},n+1,0],IntegerDigits[c]]]]; Array[sp0,20] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Aug 30 2020 *)
%Y A139535 Cf. A037053, A003617, A139536.
%K A139535 nonn,base
%O A139535 1,1
%A A139535 _Lekraj Beedassy_, Apr 25 2008