This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A264776 #21 Jul 14 2019 22:24:42 %S A264776 1,2,47,160,6070026,47418729166667,4741872916666741666666666667, %T A264776 47418729166667416666666666674166666666666666666666666667 %N A264776 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is pentagonal: (3n^2 - n)/2. %C A264776 It appears that a(n) = ceiling((a(n-1) + 5/12)*10^(7*2^(n-6))) for n >= 7. - _Jon E. Schoenfield_, Nov 24 2015 %H A264776 Wikipedia, <a href="https://en.wikipedia.org/wiki/Pentagonal_number">Pentagonal number</a> %e A264776 1, 12, 1247, 1247160, 12471606070026 are pentagonal. %o A264776 (PARI) ispentagonal(n)=ispolygonal(n,5) %o A264776 first(m)=my(v=vector(m),s="");s="1";v[1]=1;for(i=2,m,n=1;while(!ispentagonal(eval(concat(s,Str(n)))),n++);v[i]=n;s=concat(s,Str(n)));v %Y A264776 Cf. A000326, A264738. %K A264776 nonn,base,more %O A264776 1,2 %A A264776 _Anders Hellström_, Nov 24 2015 %E A264776 a(6)-a(8) from _Jon E. Schoenfield_, Nov 24 2015