A074821 Numbers k such that tau(k) = tau(2k+1).

2, 3, 4, 5, 10, 11, 23, 27, 29, 34, 38, 41, 46, 53, 55, 57, 62, 76, 77, 83, 89, 91, 93, 106, 113, 118, 123, 129, 131, 133, 136, 143, 145, 159, 161, 173, 177, 179, 185, 191, 201, 203, 205, 206, 212, 213, 218, 226, 232, 233, 235, 239, 251, 259, 267, 281, 291, 293

Offset: 1

Author

Benoit Cloitre, Sep 08 2002

Keywords

• nonn

Comments

If a term is a perfect square, then its square root must be in A001542. The first few such terms are the squares of 2, 2378, 93222358, 165326326037771920630... - Ivan Neretin, May 25 2016

Links

• Ivan Neretin, Table of n, a(n) for n = 1..10014

Crossrefs

Cf. A000005 (tau), A001542, A005384 (subsequence), A099774.

Programs

Mathematica

Select[Range[300], DivisorSigma[0, #] == DivisorSigma[0, 2 # + 1] &] (* Ivan Neretin, May 25 2016 *)

PARI

isok(k) = numdiv(k) == numdiv(2*k+1); \\ Amiram Eldar, May 08 2025