cp's OEIS Frontend

A109428 Numbers n such that sigma(n)/omega(n) is not an integer [sigma(n) =sum of divisors of n; omega(n)=number of distinct prime factors of n].

%I A109428 #7 Aug 09 2012 09:03:43
%S A109428 18,36,50,72,84,98,100,144,156,162,196,200,225,228,242,252,273,288,
%T A109428 300,324,336,338,364,372,392,399,400,441,444,468,484,516,525,532,576,
%U A109428 578,624,648,651,676,684,700,722,732,741,756,777,784,800,804,819,868,876
%N A109428 Numbers n such that sigma(n)/omega(n) is not an integer [sigma(n) =sum of divisors of n; omega(n)=number of distinct prime factors of n].
%C A109428 Integers greater than 1 and not in A109427.
%H A109428 Enrique Pérez Herrero, <a href="/A109428/b109428.txt">Table of n, a(n) for n = 1..5000</a>
%e A109428 The number 36 is in the sequence because sigma(36)=91 (1+2+3+4+6+9+12+18+36) and omega(36)=2 (2,3) and so sigma(36)/omega(36)=91/2.
%e A109428 The number 12 is not in the sequence because sigma(12)=28 (1+2+3+4+6+12) and omega(12)=2 (2,3) and so sigma(12)/omega(12)=14.
%p A109428 with(numtheory): b:=proc(n) if type(sigma(n)/nops(factorset(n)),integer)=false then n else fi end: seq(b(n),n=2..1000);
%t A109428 Select[Range[2,1000],!IntegerQ[DivisorSigma[1,#]/PrimeNu[#]]&] (* _Harvey P. Dale_, Aug 09 2012 *)
%Y A109428 Cf. A109427.
%K A109428 nonn
%O A109428 1,1
%A A109428 _Emeric Deutsch_, Jun 28 2005