A015730 Numbers n such that tau(sigma(n))= tau(tau(n)).

1, 2, 4, 9, 16, 18, 25, 50, 64, 144, 289, 576, 578, 729, 1458, 1600, 1681, 2401, 2916, 3362, 3481, 3600, 4096, 4624, 4802, 5041, 6962, 7921, 9604, 10082, 10201, 11664, 15625, 15842, 17161, 18225, 18496, 20402, 21609, 24400, 26896, 27889

Offset

1,2

Links

G. C. Greubel, Table of n, a(n) for n = 1..475

Mathematica

Do[ If[ DivisorSigma[ 0, DivisorSigma[ 0, n ] ] == DivisorSigma[ 0, DivisorSigma[ 1, n ] ], Print[ n ] ], {n, 1, 50000} ]

Prog

(PARI) isok(n) = numdiv(sigma(n)) == numdiv(numdiv(n)); \\ Michel Marcus, Aug 12 2017

Crossrefs

Sequence in context: A014290 A337343 A379112 * A073858 A006474 A110878

Adjacent sequences: A015727 A015728 A015729 * A015731 A015732 A015733

Keyword

nonn,easy

Author

Robert G. Wilson v

Status

approved