A069953 - OEIS

%I #10 Nov 08 2017 12:54:06

%S 10,28,51,170,243,246,946,1128,1281,1353,1695,2431,4336,4411,4625,

%T 4803,7501,7936,8625,11914,12545,13255,15051,16385,24643,27870,29404,

%U 31828,33490,36751,39676,47046,52801,53291,55471,58807,66249,72076,74061,81676,90775

%N Composite n such that (n-1)*phi(n) is a perfect square.

%C If k>1 k*phi(k) is not a perfect square. Trivially, if p is prime (p-1)*phi(p) is a perfect square.

%H Donovan Johnson, <a href="/A069953/b069953.txt">Table of n, a(n) for n = 1..1000</a>

%t psQ[n_]:=CompositeQ[n]&&IntegerQ[Sqrt[(n-1)EulerPhi[n]]]; Select[ Range[ 100000],psQ] (* _Harvey P. Dale_, Nov 08 2017 *)

%o (PARI) for(n=2,120000,if(frac(sqrt((n-1+isprime(n))*eulerphi(n)))==0,print1(n,",")))

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, Apr 27 2002