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 A166955 #9 May 13 2013 01:54:10 %S A166955 1,2,5,8,10,12,15,16,17,20,24,30,32,34,37,40,48,51,57,60,63,64,68,74, %T A166955 76,80,85,96,101,102,108,114,120,125,126,128,136,160,170,185,192,197, %U A166955 202,204,219,240,247,250,255,256,257,259,272,273,285,292,296,304,315,320,327 %N A166955 phi(n) is a perfect power. %H A166955 Charles R Greathouse IV, <a href="/A166955/b166955.txt">Table of n, a(n) for n = 1..10000</a> %e A166955 a(1)=1 (phi(1)=1^1); a(2)=2 (phi(2)=1^1); a(3)=5 (phi(5)=2^2). %t A166955 ppQ[n_] := GCD @@ Last /@ FactorInteger@ n > 1; ppQ[1] = True; Select[ Range@ 330, ppQ[ EulerPhi[ #]] &] (* _Robert G. Wilson v_, Dec 12 2012 *) %o A166955 (PARI) v=[1,2]; for(n=3, 320, if(ispower(eulerphi(n)), v=concat(v,n))); v (Hobson) %Y A166955 Essentially the same as A065528. %K A166955 nonn,easy %O A166955 1,2 %A A166955 _Juri-Stepan Gerasimov_, Oct 25 2009 %E A166955 b-file from _Charles R Greathouse IV_, Mar 25 2010