A067781 Numbers k such that phi(k) and sigma(k) are both perfect squares.

1, 170, 364, 679, 5044, 5130, 5670, 5770, 8721, 8736, 9154, 9639, 9809, 14322, 16376, 22413, 27783, 30256, 32025, 37114, 38760, 51455, 71604, 78570, 82615, 88392, 92004, 100821, 101153, 104168, 115430, 121056, 133569, 139954, 148568, 171069, 177940, 198462, 217868

Offset

1,2

Comments

A subsequence of A011257. - M. F. Hasler, Sep 22 2009

Links

Donovan Johnson, Table of n, a(n) for n = 1..5000

Formula

Equals A006532 intersect A039770. - M. F. Hasler, Sep 22 2009

Mathematica

Select[Range[250000], And @@ (IntegerQ[Sqrt[#1]] & /@ {EulerPhi[#], DivisorSigma[1, #]} ) &] (* Amiram Eldar, May 08 2025 *)

Prog

(PARI) isok(k) = {my(f = factor(k)); issquare(eulerphi(f)) && issquare(sigma(f)); } \\ Amiram Eldar, May 08 2025

Crossrefs

Cf. A000010 (phi), A000203 (sigma), A006532, A011257, A039770.

Sequence in context: A043762 A043771 A158221 * A043344 A045153 A045041

Adjacent sequences: A067778 A067779 A067780 * A067782 A067783 A067784

Keyword

nonn

Author

Benoit Cloitre, Feb 07 2002

Status

approved