A060668 - OEIS

%I #8 Mar 03 2014 23:07:26

%S 8,20,220,272,300,368,416,456,500,656,732,848,876,1092,1160,1212,1236,

%T 1328,1376,1424,1568,1624,1716,1808,2144,2244,2336,2420,2460,2480,

%U 2528,2556,2768,3056,3080,3252,3320,3344,3536,3560,3612,3728,3732,3900,4016

%N Numbers n such that phi(x) = n has exactly 5 solutions.

%H T. D. Noe, <a href="/A060668/b060668.txt">Table of n, a(n) for n = 1..1000</a>

%e 8 = phi(15) = phi(16) = phi(20) = phi(24) = phi(30).

%t a = Table[ 0, {5000} ]; Do[ p = EulerPhi[ n ]; If[ p < 5001, a[[ p ]]++ ], {n, 1, 15000} ]; Select[ Range[ 5000 ], a[[ # ]] == 5 & ]

%o (PARI) is(n)=sum(k=1,n,eulerphi(k)==n)==5 \\ _Charles R Greathouse IV_, Mar 03 2014

%Y Cf. A007366, A007367.

%K nonn

%O 1,1

%A _Robert G. Wilson v_, Apr 18 2001