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 A303149 #6 May 26 2018 22:09:00 %S A303149 1,18,1,2,13,14,15,16,17,18,1,2,13,14,15,16,17,18,1,2,3,4,5,6,7,8,9, %T A303149 10,11,12,13,14,15,16,17,18,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17, %U A303149 18,1,2,3,0,0,0,0,0,0,0,0,0,0,0,0,0,0,4,5,6,7,8,9 %N A303149 Group number of n-th element in the periodic table, or 0 if the element does not have a group number. %C A303149 a(n) is 0 for the f-block elements. %H A303149 Wikipedia, <a href="https://en.wikipedia.org/wiki/Block_(periodic_table)#f-block">Block (periodic table) - f-block</a> %H A303149 Wikipedia, <a href="https://en.wikipedia.org/wiki/Group_(periodic_table)">Group (periodic table)</a> %H A303149 Wikipedia, <a href="https://en.wikipedia.org/wiki/List_of_chemical_elements">List of chemical elements</a> %H A303149 Wikipedia, <a href="https://en.wikipedia.org/wiki/Periodic_table">Periodic table</a> %Y A303149 Cf. A138718. %K A303149 nonn,fini %O A303149 1,2 %A A303149 _Felix Fröhlich_, Apr 22 2018