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 A071181 #26 Nov 17 2024 04:54:56
%S A071181 2,2,5,5,9,5,9,9,9,9,7,9,19,9,14,14,20,9,13,14,19,7,7,14,16,19,13,19,
%T A071181 11,14,13,20,16,20,34,19,31,13,34,20,30,19,13,16,34,7,7,20,13,16,27,
%U A071181 34,11,13,30,34,31,11,7,20,37,13,31,27,51,16,13,27,14,34,9,34,63,31,30,31
%N A071181 Number of numbers k such that phi(k) divides phi(n).
%H A071181 Amiram Eldar, <a href="/A071181/b071181.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..2048 from Antti Karttunen computed with the help of b-file of A057826 provided by T. D. Noe)
%H A071181 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%F A071181 For all n, a(n) >= A066412(n). - _Antti Karttunen_, Jul 17 2017
%o A071181 (PARI) for(n=1,100,print1(sum(i=1,1000,if(eulerphi(n)%eulerphi(i),0,1)),","))
%o A071181 (PARI) a(n) = {my(v = 0); fordiv(eulerphi(n), d, v += invphiNum(d)); v;} \\ _Amiram Eldar_, Nov 12 2024, using _Max Alekseyev_'s invphi.gp; edited by _Max Alekseyev_, Nov 16 2024
%o A071181 (Scheme)
%o A071181 ;; A naive implementation requiring precomputed A057826:
%o A071181 (define (A071181 n) (if (<= n 2) 2 (let ((ph (A000010 n))) (let loop ((k (A057826 (/ ph 2))) (s 0)) (if (zero? k) s (loop (- k 1) (+ s (if (zero? (modulo ph (A000010 k))) 1 0)))))))) ;; _Antti Karttunen_, Jul 18 2017
%Y A071181 Cf. A000010, A057826, A066412.
%K A071181 easy,nonn
%O A071181 1,1
%A A071181 _Benoit Cloitre_, Jun 10 2002