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 A039771 #29 Sep 01 2025 20:21:28 %S A039771 1,2,15,16,20,24,30,85,128,136,160,170,192,204,240,247,259,327,333, %T A039771 351,399,405,436,494,518,532,648,654,666,684,702,756,771,798,810,1024, %U A039771 1028,1088,1111,1255,1280,1360,1375,1536,1542,1632,1843,1853,1875,1920,2008 %N A039771 Numbers k such that phi(k) is a perfect cube. %C A039771 a(n) is prime only for a(2)=2, for other cases: eulerphi(p) = p-1 = n^3, and p = 1 + n^3 = (n+1)(n^2-n+1), so p cannot be a prime. - _Enrique Pérez Herrero_, Aug 29 2010 %C A039771 A013730 is a subsequence. - _Enrique Pérez Herrero_, Aug 29 2010 %H A039771 Amiram Eldar, <a href="/A039771/b039771.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..2500 from E. Pérez Herrero) %e A039771 phi(247) = 216 = 6*6*6. %p A039771 q:= n-> (c-> iroot(c, 3)^3=c)(numtheory[phi](n)): %p A039771 select(q, [$1..2200])[]; # _Alois P. Heinz_, Sep 01 2025 %t A039771 Select[ Range[ 2000 ], IntegerQ[ Power[ EulerPhi[ # ], 1/3 ] ]& ] %o A039771 (PARI) for(n=1,1e4,if(ispower(eulerphi(n),3),print1(n", "))) \\ _Charles R Greathouse IV_, Jul 31 2011 %Y A039771 Cf. A000010, A007614, A039770, A216412. %K A039771 nonn,changed %O A039771 1,2 %A A039771 _Olivier Gérard_