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 A072455 #20 Nov 07 2024 08:33:15
%S A072455 1,2,3,4,4,6,7,4,8,9,7,11,10,8,13,14,9,11,16,10,17,18,9,20,19,13,22,
%T A072455 17,15,25,26,14,21,28,16,29,30,14,23,31,19,33,27,19,35,28,22,29,37,19,
%U A072455 38,39,16,41,42,26,44,33,26,38,41,27,36,47,29,49,43,22,51,52,32,43,40,27
%N A072455 Number of totients in the reduced residue system of 2n-1.
%H A072455 Amiram Eldar, <a href="/A072455/b072455.txt">Table of n, a(n) for n = 1..10000</a>
%F A072455 a(n) = phi(2*n-1) - A072454(n). [Corrected by _Sean A. Irvine_, Oct 04 2024]
%e A072455 For n=31: reduced residue system(31) = {1,...,30} with 15 odd and 15 even numbers. From the odd terms only the term 1 is totient, while from the 15 even terms, 2 terms, {14,26}, are nontotients, so 13 terms are totients. All totients count 1 + 13 = 14, thus a((31+1)/2) = a(16) = 14.
%o A072455 (PARI) a(n) = {my(m = 2*n-1); sum(k = 1, m, gcd(m, k) == 1 && istotient(k));} \\ _Amiram Eldar_, Nov 07 2024
%Y A072455 Cf. A000010, A002202, A005277, A037225, A072451, A072454.
%K A072455 nonn
%O A072455 1,2
%A A072455 _Labos Elemer_, Jun 19 2002