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 A319213 #25 Dec 09 2024 05:28:40
%S A319213 2,4,12,20,56,36,144,96,216,144,520,240,840,480,576,816,1568,756,2520,
%T A319213 1232,1872,1560,4400,1440,4320,3024,4860,3168,7056,2640,9000,5984,
%U A319213 7920,6144,10080,4752,17784,7992,13104,9184,22080,7560,23688,12960,14688,15840,33120
%N A319213 a(n) = phi(n^3 - 1)/3 where phi is A000010.
%H A319213 Seiichi Manyama, <a href="/A319213/b319213.txt">Table of n, a(n) for n = 2..1000</a>
%H A319213 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>.
%H A319213 Wikipedia, <a href="https://en.wikipedia.org/wiki/Euler%27s_totient_function">Euler's totient function</a>.
%F A319213 Sum_{k=1..n} a(k) = c * n^4 + O((n*log(n))^3), where c = (2/27) * Product_{p prime == 1 (mod 3)} (1 - 3/p^2) * Product_{p prime == 2 (mod 3)} (1 - 1/p^2) = 0.047313356295... . - _Amiram Eldar_, Dec 09 2024
%t A319213 EulerPhi[Range[2, 50]^3 - 1]/3 (* _Paolo Xausa_, Jun 18 2024 *)
%o A319213 (PARI) {a(n) = eulerphi(n^3-1)/3}
%Y A319213 Row 3 of A369291.
%Y A319213 Cf. A000010, A068601 (n^3-1).
%Y A319213 phi(n^b - 1)/b: A319210 (b=2), this sequence (b=3), A319214 (b=5).
%K A319213 nonn,easy
%O A319213 2,1
%A A319213 _Seiichi Manyama_, Sep 13 2018