A138171 Odd n where d(n) > d(n+1), where d(n) = number of positive divisors of n.

45, 81, 105, 117, 165, 225, 261, 273, 297, 315, 325, 333, 345, 357, 385, 405, 435, 441, 465, 477, 495, 513, 525, 555, 561, 567, 585, 595, 621, 625, 627, 651, 675, 693, 705, 715, 765, 777, 795, 801, 825, 837, 855, 861, 885, 891, 897, 915, 925, 945, 957, 975

Offset

1,1

Comments

Terms calculated by M. F. Hasler.

First term == 5 (mod 6) is a(385) = 6125. - Jianing Song, Apr 03 2018

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Maple

with(numtheory): a:=proc(n) if tau(2*n)<tau(2*n-1) then 2*n-1 else end if end proc: seq(a(n), n=1..500); # Emeric Deutsch, Mar 12 2008
with(numtheory): a:=proc (n) if `mod`(n, 2)=1 and tau(n+1) < tau(n) then n else end if end proc: seq(a(n), n=1..1000); # Emeric Deutsch, Mar 31 2008

Mathematica

Select[Range[1, 1001, 2], DivisorSigma[0, #]>DivisorSigma[0, #+1]&] (* Harvey P. Dale, Jul 08 2017 *)

Prog

(PARI) isok(n) = (n%2) && (numdiv(n) > numdiv(n+1)); \\ Michel Marcus, Apr 04 2018
(PARI) lista(nn) = forstep(n=1, nn, 2, if(numdiv(n) > numdiv(n+1), print1(n, ", "))); \\ Altug Alkan, Apr 04 2018
(GAP) Filtered([1, 3..1301], n->Tau(n)>Tau(n+1)); # Muniru A Asiru, Apr 05 2018

Crossrefs

Cf. A138046, A138172.

Sequence in context: A364718 A102578 A026060 * A305154 A280407 A063343

Adjacent sequences: A138168 A138169 A138170 * A138172 A138173 A138174

Keyword

nonn

Author

Leroy Quet, Mar 03 2008

Status

approved