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 A058541 #9 Jul 16 2017 05:09:13 %S A058541 1,2,3,5,9,14,18,15,21,31,61,121,230,258,246,234,156,76,88,84,28,28, %T A058541 28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28, %U A058541 28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28 %N A058541 Trajectory of 1 under map that sends x to 3x - sigma(x). %t A058541 NestList[3#-DivisorSigma[1,#]&,1,70] (* _Harvey P. Dale_, Jul 23 2012 *) %Y A058541 Cf. A037159, A058542, A058545. %K A058541 nonn %O A058541 0,2 %A A058541 _N. J. A. Sloane_, _Jud McCranie_, Dec 24 2000