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 A247900 #16 Sep 02 2020 02:58:52 %S A247900 54,36,54,72,18,18,36,36,18,18,54,36,18,36,54,36,54,18,18,18,18,54,18, %T A247900 54,18,36,54,18,54,18,36,18,36,54,18,18,18,18,54,18,18,18,18,18,36,36, %U A247900 18,18,72,18,36,36,36,18,36,54,18,18,36,18,18,18,18,18,54,36,36,18,72,18,18,72,18,36,36,18,18,18,36,90 %N A247900 First differences of A247681. %C A247900 All terms have digital root equal to 9. %H A247900 Odimar Fabeny, <a href="/A247900/b247900.txt">Table of n, a(n) for n = 1..9999</a> [first term removed by _Georg Fischer_, Sep 02 2020] %t A247900 Differences[Select[Range[1,3500,18],!PrimeQ[#]&]] (* _Harvey P. Dale_, Apr 25 2018 *) %Y A247900 Cf. A247681. %K A247900 nonn,easy %O A247900 1,1 %A A247900 _Odimar Fabeny_, Sep 26 2014 %E A247900 First term 54 removed by _Georg Fischer_, Sep 02 2020