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 A372621 #19 Feb 16 2025 08:34:06
%S A372621 1,2,5,7,11,14,20,24,33,37,47,53,65,71,83,91,107,116,134,142,160,170,
%T A372621 192,204,224,236,263,275,303,315,345,361,391,407,431,449,485,503,539,
%U A372621 555,595,613,655,675,711,733,779,803,845,865,913,937,989,1016,1056,1080,1134
%N A372621 a(n) = (1/2) * Sum_{k=1..n} phi(3*k).
%H A372621 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>.
%F A372621 a(n) ~ (27/(8*Pi^2)) * n^2. - _Amiram Eldar_, May 08 2024
%t A372621 Accumulate[Table[EulerPhi[3*n], {n, 1, 60}]]/2 (* _Amiram Eldar_, May 08 2024 *)
%o A372621 (PARI) a(n) = sum(k=1, n, eulerphi(3*k))/2;
%Y A372621 Equals A194881 - 1.
%Y A372621 Partial sums of A195459.
%Y A372621 Column k=3 of A372619.
%Y A372621 Cf. A000010, A194881.
%K A372621 nonn
%O A372621 1,2
%A A372621 _Seiichi Manyama_, May 07 2024