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%I #23 Jan 23 2017 13:08:10
%S 1,3,36,54765,123152388,374848814886363636,
%T 85794018663817263665487289502938826,
%U 107072047880615405294526336549204869795454545454545454545454545454545466
%N a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 13-gonal: (11n^2 - 9n)/2.
%H Jon E. Schoenfield, <a href="/A264842/b264842.txt">Table of n, a(n) for n = 1..11</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Polygonal_number">Polygonal number</a>
%e 1, 13, 1336, 133654765 are 13-gonal.
%o (PARI) tridecagonal(n)=ispolygonal(n, 13)
%o first(m)=my(s=""); s="1"; print1(1, ", "); for(i=2, m, n=1; while(!tridecagonal(eval(concat(s, Str(n)))), n++); print1(n, ", "); s=concat(s, Str(n)))
%Y Cf. A051671, A051865 (13-gonal numbers), A061109, A061110, A261696, A264733, A264738, A264776, A264777, A264848, A264849, A264804.
%K nonn,base
%O 1,2
%A _Anders Hellström_, Nov 26 2015
%E More terms from _Jon E. Schoenfield_, Nov 27 2015