A190276 Numbers k such that tau(2k-1) = tau(2k+1) where tau(k) = A000005(k).

2, 3, 6, 9, 15, 17, 21, 28, 30, 36, 43, 46, 47, 51, 54, 62, 69, 71, 72, 75, 80, 90, 92, 93, 96, 99, 101, 102, 107, 108, 109, 110, 114, 118, 120, 122, 124, 133, 135, 141, 150, 151, 152, 156, 160, 161, 164, 170, 171, 174, 196, 197, 206, 207, 208, 210, 212, 216, 223, 226, 231, 235, 236, 238

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Harvey P. Dale)

Formula

A000005(2*a(n)-1) = A000005(2*a(n)+1).

a(n) = (A066692(n) + 1)/2. - Michel Marcus, Jan 04 2016

Mathematica

nn=250; With[{tau=DivisorSigma[0, Range[2nn+1]]}, Select[Range[nn], tau[[2#-1]] == tau[[2#+1]]&]] (* Harvey P. Dale, May 19 2012 *)

Prog

(Magma) [n: n in [1..300] | NumberOfDivisors(2*n-1) eq NumberOfDivisors(2*n+1)]; // Vincenzo Librandi, Jan 03 2016
(PARI) isok(n) = numdiv(2*n-1)==numdiv(2*n+1); \\ Michel Marcus, Jan 04 2016

Crossrefs

Cf. A000005, A066692.

Sequence in context: A032231 A200273 A303664 * A113808 A337114 A308870

Adjacent sequences: A190273 A190274 A190275 * A190277 A190278 A190279

Keyword

nonn,easy

Author

Juri-Stepan Gerasimov, May 07 2011

Extensions

Corrected by Harvey P. Dale, May 19 2012

Status

approved