A073568 - OEIS

%I #10 Dec 15 2017 17:35:58

%S 3,4,5,6,8,12,20,24,32,40,44,50,60,72,80,84,96,144,156,162,180,192,

%T 208,224,240,244,272,288,312,316,336,396,404,420,432,464,480,512,516,

%U 524,596,624,648,684,704,736,804

%N Number of anti-divisors of n divides the number of divisors of n.

%C See A066272 for definition of anti-divisor.

%H Charles R Greathouse IV, <a href="/A073568/b073568.txt">Table of n, a(n) for n = 1..10000</a>

%t atQ[n_] := Divisible[DivisorSigma[0, n], Count[Flatten[Quotient[#, Rest[Select[Divisors[#], OddQ]]] & /@ (2 n + Range[-1, 1])], Except[1]]]; Select[Range[3, 805], atQ] (* _Jayanta Basu_, Jul 01 2013 *)

%o (PARI) ad(n)=numdiv(2*n+1)+numdiv(2*n-1)+numdiv(n>>valuation(n, 2))-5

%o is(n)=n>2 && numdiv(n)%ad(n)==0 \\ _Charles R Greathouse IV_, Jul 01 2013

%Y Cf. A066272, A000005.

%K nonn

%O 1,1

%A _Jason Earls_, Aug 31 2002