A015727 - OEIS

%I #17 Sep 08 2022 08:44:40

%S 8,15,24,28,35,40,42,87,92,126,130,143,145,150,174,184,273,323,369,

%T 374,378,396,847,899,957,984,1134,1136,1220,1236,1245,1368,1448,1449,

%U 1763,1812,1892,2167,2456,2494,3402,3415,3599,4023,4098,4623

%N Numbers n such that phi(n) * sigma(n) + 4 is a perfect square.

%H Vincenzo Librandi, <a href="/A015727/b015727.txt">Table of n, a(n) for n = 1..200</a>

%H Richard K. Guy, <a href="http://www.jstor.org/stable/2974586">Divisors and desires</a>, Amer. Math. Monthly, 104 (1997), 359-360.

%t Select[Range[5000],IntegerQ[Sqrt[EulerPhi[#]DivisorSigma[1,#]+4]]&] (* _Harvey P. Dale_, Mar 12 2014 *)

%o (Magma) [n: n in [1..5000] | IsSquare(EulerPhi(n) * SumOfDivisors(n) + 4)]; // _Vincenzo Librandi_, Mar 14 2014

%K nonn

%O 1,1

%A _Robert G. Wilson v_