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a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 11-gonal: (9n^2 - 7n)/2.
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%I #17 Jan 26 2024 12:47:40

%S 1,1,526,64095,21420730041,4528059468080555555556,

%T 3834345160635370971474665069772601398563211,

%U 100751687713984558500838936986634939491022212000570658953744730444103042117925197608458

%N a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 11-gonal: (9n^2 - 7n)/2.

%H Chai Wah Wu, <a href="/A264804/b264804.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>

%o (PARI) hendecagonal(n)=ispolygonal(n,11)

%o first(m)=my(v=vector(m),s="");s="1";print1(1, ", ");for(i=2,m,n=1;while(!hendecagonal(eval(concat(s,Str(n)))),n++);print1(n, ", ");s=concat(s,Str(n)))

%Y Cf. A051671, A051682 (11-gonal numbers), A061109, A061110, A261696, A264733, A264738, A264776, A264777, A264842, A264848, A264849.

%K nonn,base

%O 1,3

%A _Anders Hellström_, Nov 25 2015

%E a(5)-a(8) from _Chai Wah Wu_, Mar 16 2018