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a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is a heptagonal number (A000566).
7

%I #32 Dec 20 2015 13:46:32

%S 1,8,9,5296,9701946,3254409476919,214948107829163565920446430,

%T 31021083401526077498957998513210666182754941982955902,

%U 81969637737594122848099953820072074027662186843090573669278105078583525219219292274950391247747733666835448

%N a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is a heptagonal number (A000566).

%C a(20) has 219815 decimal digits. - _Chai Wah Wu_, Nov 25 2015

%H Chai Wah Wu, <a href="/A264777/b264777.txt">Table of n, a(n) for n = 1..12</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Heptagonal_number">Heptagonal number</a>

%e 1, 18, 189, 1895296 are heptagonal.

%o (PARI) isheptagonal(n)=ispolygonal(n,7)

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

%Y Cf. A000566, A264738.

%K nonn,base

%O 1,2

%A _Anders Hellström_, Nov 24 2015

%E a(6)-a(8) from _Jon E. Schoenfield_, Nov 24 2015

%E a(9) from _Chai Wah Wu_, Nov 25 2015