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 A175189 #23 Oct 13 2024 01:28:08
%S A175189 7,33,29,59,347,2039,4079,32633,65267,913739,1827479,36549581
%N A175189 Smallest integer m such that phi(phi(m))^n + tau(phi(m))^n = phi(rad(m))^n, where n is the number of iterations of phi(phi), tau(phi) and phi(rad) functions.
%C A175189 Remarks about an interesting property of the equation phi(phi(m))^k + tau(phi(m))^k = phi(rad(m))^k: Let p be a prime number. If p is a solution of this equation with k iterations, and if q = 2*p+1 is prime, then q is solution of the equation with k+1 iterations.
%C A175189 Proof: we use the following properties, if p is prime: phi(phi(2*p+1)) = phi(2*p) = p-1; tau(phi(2*p+1)) = tau(2*p) = 4; phi(rad(2*p+1)) = phi(2*p+1) = 2*p; phi(phi(p)) = phi(p-1); tau(phi(p)) = tau(p-1); phi(rad(p)) = phi(p) = p-1.
%C A175189 Example: 2039 is prime and is solution for k = 6, and 4079 = 2*2039 + 1 is prime and is solution for n = 7; idem with the primes 32633, 913739, but p = 36549581 is prime and solution for 12 iterations, but 2*p + 1 is not prime, so it is not a solution.
%D A175189 M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.
%H A175189 M. Abramowitz and I. A. Stegun, eds., <a href="http://www.convertit.com/Go/ConvertIt/Reference/AMS55.ASP">Handbook of Mathematical Functions, National Bureau of Standards</a>, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
%H A175189 C. K. Caldwell, <a href="https://t5k.org/glossary/page.php?sort=Tau">The Prime Glossary, Number of divisors</a>
%H A175189 Wikipedia, <a href="http://en.wikipedia.org/wiki/Euler's_phi_function">Euler's totient function</a>
%e A175189 For n=1, phi(phi(7)) = 2, tau(phi(7)) = 4, phi(rad(7)) = phi(7) = 6, then 2 + 4 = 6.
%e A175189 For n=2, phi(phi(phi(phi(33)))) = 2, tau(phi(tau(phi(33)))) = 2, phi(rad(phi(rad(33)))) = 4, then 2 + 2 = 4.
%e A175189 For n=3, phi(phi(phi(phi(phi(phi(29)))))) = 1, tau(phi(tau(phi(tau(phi(29)))))) = 1, phi(rad(phi(rad(phi(rad(29)))))) = 2, then 1 + 1 = 2.
%p A175189 with(numtheory):for n from 1 to 100 do:indic:=0:for x from 1 to 10000 while(indic=0 ) do:x0:=x:y0:=x:z0:=x: for iter from 1 to n do:x1:=phi(phi(x0)): y1:= tau(phi(y0)): zz1:= ifactors(z0)[2] : zz2 :=mul(zz1[i][1], i=1..nops(zz1)): z1:=phi(zz2):x0:=x1:y0:=y1:z0:=z1:od :if x0 +y0=z0 then print (x):indic:=1:else fi:od:od:
%o A175189 (PARI) rad(m) = factorback(factorint(m)[, 1]); \\ A007947
%o A175189 phi_phi(m,n) = {for (k=1, n, m = eulerphi(eulerphi(m));); m;}
%o A175189 tau_phi(m,n) = {for (k=1, n, m = numdiv(eulerphi(m));); m;}
%o A175189 phi_rad(m,n) = {for (k=1, n, m = eulerphi(rad(m));); m;}
%o A175189 a(n) = {my(m=1); while (phi_phi(m,n)+ tau_phi(m,n) != phi_rad(m,n), m++); m;} \\ _Michel Marcus_, Sep 17 2020
%Y A175189 Cf. A000010 (phi), A007947 (rad), A000005 (tau), A002183.
%K A175189 nonn,more
%O A175189 1,1
%A A175189 _Michel Lagneau_, Mar 01 2010
%E A175189 Edited by _Michel Marcus_, Sep 17 2020