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 A271553 #6 Jan 11 2020 15:57:47
%S A271553 1,2,4,9,64
%N A271553 a(n) = H_{n-1}(n,n).
%C A271553 H is the hyperoperation function see A054871 for more information.
%C A271553 This sequence is a shifted version of the weak Ackermann numbers H_n(n,n).
%Y A271553 Cf. A189896 (H_n(n,n)), A054871.
%K A271553 nonn,bref
%O A271553 0,2
%A A271553 _Natan Arie Consigli_, Apr 09 2016