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 A372639 #13 Feb 16 2025 08:34:06
%S A372639 1,3,5,9,14,18,24,32,38,48,58,66,78,90,100,116,132,144,162,182,194,
%T A372639 214,236,252,277,301,319,343,371,391,421,453,473,505,535,559,595,631,
%U A372639 655,695,735,759,801,841,871,915,961,993,1035,1085,1117,1165,1217,1253,1303
%N A372639 a(n) = (1/4) * Sum_{k=1..n} phi(10*k).
%H A372639 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>.
%F A372639 a(n) ~ (25/(6*Pi^2)) * n^2. - _Amiram Eldar_, May 08 2024
%t A372639 Accumulate[Table[EulerPhi[10*n], {n, 1, 60}]]/4 (* _Amiram Eldar_, May 08 2024 *)
%o A372639 (PARI) a(n) = sum(k=1, n, eulerphi(10*k))/4;
%Y A372639 Column k=10 of A372619.
%Y A372639 Cf. A000010.
%K A372639 nonn
%O A372639 1,2
%A A372639 _Seiichi Manyama_, May 08 2024