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%I #6 May 26 2018 22:09:00
%S 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 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 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 Group number of n-th element in the periodic table, or 0 if the element does not have a group number.
%C a(n) is 0 for the f-block elements.
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Block_(periodic_table)#f-block">Block (periodic table) - f-block</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Group_(periodic_table)">Group (periodic table)</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/List_of_chemical_elements">List of chemical elements</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Periodic_table">Periodic table</a>
%Y Cf. A138718.
%K nonn,fini
%O 1,2
%A _Felix Fröhlich_, Apr 22 2018