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%I #26 Mar 17 2018 18:43:51
%S 1,9,224,631909,58000804596,61688194098028272863216,
%T 2514637794509678630513616176470588235294117671,
%U 941048382372874985200592647058823529411764708485294117647058823529411764705882352941176469
%N a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 19-gonal: (17n^2 - 15n)/2.
%H Chai Wah Wu, <a href="/A264848/b264848.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, 19, 19224, 19224631909 are 19-gonal.
%o (PARI) enneadecagonal(n)=ispolygonal(n, 19)
%o first(m)=my(s=""); s="1"; print1(1, ", "); for(i=2, m, n=1; while(!enneadecagonal(eval(concat(s, Str(n)))), n++); print1(n, ", "); s=concat(s, Str(n)))
%Y Cf. A051671, A051871 (19-gonal numbers), A061109, A061110, A261696, A264733, A264738, A264776, A264777, A264804, A264842, A264849.
%K nonn,base
%O 1,2
%A _Anders Hellström_, Nov 26 2015
%E a(5)-a(8) from _Chai Wah Wu_, Mar 16 2018
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