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 A032670 #11 Sep 16 2020 12:46:01 %S A032670 1,3,9,19,27,33,39,43,51,57,61,69,73,87,97,103,109,121,127,129,151, %T A032670 159,177,187,193,207,211,213,219,229,237,243,247,253,283,297,307,309, %U A032670 321,331,333,349,351,369,393,411,417,433,451,457,459,477,481 %N A032670 Digit '7' concatenated with a(n) is a prime. %H A032670 Seiichi Manyama, <a href="/A032670/b032670.txt">Table of n, a(n) for n = 1..10000</a> %t A032670 Select[Range[500],PrimeQ[7*10^IntegerLength[#]+#]&] (* _Harvey P. Dale_, Sep 16 2020 *) %K A032670 nonn,base %O A032670 1,2 %A A032670 _Patrick De Geest_, May 15 1998