A145469 Positive integers k such that d(k) = d(k-1) + d(k-2), where d(k) is the number of divisors of k.

12, 30, 40, 54, 63, 75, 80, 88, 135, 156, 165, 174, 208, 255, 260, 279, 285, 318, 328, 368, 372, 405, 423, 455, 460, 483, 490, 495, 546, 550, 552, 555, 585, 654, 726, 732, 750, 795, 836, 846, 870, 915, 935, 940, 952, 996, 1012, 1048, 1068, 1148, 1173, 1196

Offset: 1

Author

Leroy Quet, Oct 11 2008

Keywords

• nonn

Links

• Vaclav Kotesovec, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A145470.

Programs

Magma

[n:n in [3..1500]| NumberOfDivisors(n-1)+NumberOfDivisors(n-2) eq NumberOfDivisors(n)]; // Marius A. Burtea, May 08 2019

Maple

with(numtheory): a:=proc(n) if tau(n)=tau(n-1)+tau(n-2) then n else end if end proc: seq(a(n),n=3..1300); # Emeric Deutsch, Oct 23 2008

Mathematica

Select[Range[1200],DivisorSigma[0,#]==DivisorSigma[0,#-1]+ DivisorSigma[ 0, #-2]&] (* Harvey P. Dale, Jan 26 2013 *)

PARI

isok(n) = (n>2) && (numdiv(n) == numdiv(n-1) + numdiv(n-2)); \\ Michel Marcus, May 08 2019

Extensions

Extended by Emeric Deutsch, Oct 23 2008

Name edited by Jon E. Schoenfield, May 08 2019