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 A356340 #8 Aug 05 2022 06:10:12
%S A356340 1,5,23,102,444,1909,8133,34404,144714,605920,2527348,10507978,
%T A356340 43569096,180219699,743907057,3065019864,12607648238,51783970314,
%U A356340 212412697368,870249992168,3561502879100,14560944187796,59476980459794,242741090637012,989921853052930,4034101567907172
%N A356340 a(n) = Sum_{k=1..n} binomial(2*n, n-k) * phi(k), where phi is the Euler totient function.
%F A356340 a(n) ~ 3 * sqrt(n) * 4^n / Pi^(5/2).
%t A356340 Table[Sum[Binomial[2*n, n-k]*EulerPhi[k], {k, 1, n}], {n, 1, 30}]
%o A356340 (PARI) a(n) = sum(k=1, n, binomial(2*n, n-k) * eulerphi(k)); \\ _Michel Marcus_, Aug 05 2022
%Y A356340 Cf. A000010, A002088, A306988, A351146.
%K A356340 nonn
%O A356340 1,2
%A A356340 _Vaclav Kotesovec_, Aug 04 2022