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 A099147 #22 Feb 16 2025 08:32:54
%S A099147 1,6,66,8646,149497986,44699295486614406,
%T A099147 3996054033999333969062944766851266,
%U A099147 31936895685284700329548847429175178142518023225832967407199564754246
%N A099147 Iterated hexagonal numbers, starting at 1.
%C A099147 Agrees with A097419 for n>1.
%C A099147 The next term (a(9)) has 136 digits. - _Harvey P. Dale_, Nov 24 2024
%H A099147 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexagonalNumber.html">Hexagonal Number</a>.
%F A099147 a(n) = b(n) for n<=2, a(n) = b(a(n-1)) for n>2, where b(n) = A000384(n) = n*(2*n-1), the hexagonal numbers.
%F A099147 a(1) = 1, a(2) = 6, a(n) = 2*a(n-1)^2 - a(n-1) for n>2.
%F A099147 Let H(n) = n*(2*n-1) = the n-th hexagonal number. Define A(n, k) recursively by A(1, k) = H(k), A(n, k) = A(1, A(n-1, k)) for n>1. Then a(1) = A(1, 1), a(n) = A(n-1, 2) for n>1.
%e A099147 a(4) = b(a(3)) = b(b(a(2))) = b(b(b(2))) = b(b(6)) = b(66) = 8646, where b(n) = A000384(n).
%t A099147 Join[{1},NestList[PolygonalNumber[6,#]&,6,6]] (* _Harvey P. Dale_, Nov 24 2024 *)
%o A099147 (PARI)
%o A099147 {hexagonal(n) = n*(2*n-1)}
%o A099147 {a(n) = if(n<=2,hexagonal(n),hexagonal(a(n-1)))} \\ _Klaus Brockhaus_, Jan 10 2007
%Y A099147 Cf. A000384, A097419.
%K A099147 nonn
%O A099147 1,2
%A A099147 _Jonathan Vos Post_, Nov 14 2004
%E A099147 Edited by _Klaus Brockhaus_, Jan 10 2007