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 A173618 #15 Jul 09 2019 10:58:33 %S A173618 1,4,36,54,96,200,448,1280,2700,4500,5103,9720,11264,14112,14580, %T A173618 17280,26624,32928,48000,54432,71442,75000,81648,152064,184320,187500, %U A173618 258048,307200,350000,637875,1250235,1344560,1557504,2044416,2187500,2367488,3234816 %N A173618 Numbers k such that tau(phi(k)) = rad(k). %C A173618 rad(k) is the product of the primes dividing k (A007947), tau(k) is the number of divisors of k (A000005), phi(k) is the Euler totient function (A000010). %D A173618 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 A173618 Amiram Eldar, <a href="/A173618/b173618.txt">Table of n, a(n) for n = 1..100</a> %H A173618 W. Sierpinski, <a href="http://matwbn.icm.edu.pl/ksiazki/mon/mon42/mon4204.pdf">Number Of Divisors And Their Sum</a> %H A173618 Wikipedia, <a href="http://en.wikipedia.org/wiki/Euler's_phi_function">Euler's totient function</a> %F A173618 k such that A062821(k) = A007947(k). %e A173618 phi(4) = 2, tau(2) = 2 and rad(4) = 2 phi(36) = 12, tau(12) = 6 and rad(36) = 6 %p A173618 with(numtheory):for n from 1 to 1000000 do : t1:= ifactors(n)[2] : t2 :=mul(t1[i][1], i=1..nops(t1)): if tau(phi(n))= t2 then print (n): else fi: od : %t A173618 rad[n_] := Times @@ (First@# & /@ FactorInteger[n]); Select[Range[10^5], DivisorSigma[0, EulerPhi[#]] == rad[#] &] (* _Amiram Eldar_, Jul 09 2019*) %o A173618 (PARI) isok(k) = numdiv(eulerphi(k)) == factorback(factorint(k)[, 1]); \\ _Michel Marcus_, Jul 09 2019 %Y A173618 Cf. A000005, A000010, A062069, A062821, A007947, A173326. %K A173618 nonn %O A173618 1,2 %A A173618 _Michel Lagneau_, Feb 22 2010 %E A173618 a(30)-a(37) from _Donovan Johnson_, Jul 27 2011