A015727 - OEIS

A015727

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

8, 15, 24, 28, 35, 40, 42, 87, 92, 126, 130, 143, 145, 150, 174, 184, 273, 323, 369, 374, 378, 396, 847, 899, 957, 984, 1134, 1136, 1220, 1236, 1245, 1368, 1448, 1449, 1763, 1812, 1892, 2167, 2456, 2494, 3402, 3415, 3599, 4023, 4098, 4623

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..200

Richard K. Guy, Divisors and desires, Amer. Math. Monthly, 104 (1997), 359-360.

MATHEMATICA

Select[Range[5000], IntegerQ[Sqrt[EulerPhi[#]DivisorSigma[1, #]+4]]&] (* Harvey P. Dale, Mar 12 2014 *)

PROG

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

CROSSREFS

Sequence in context: A296542 A296546 A167986 * A034104 A034114 A069826

Adjacent sequences: A015724 A015725 A015726 * A015728 A015729 A015730

KEYWORD

nonn

AUTHOR

Robert G. Wilson v

STATUS

approved