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A093890 Number of primes arising as the sum of one or more divisors of n. 7

%I #10 Nov 17 2018 21:50:39

%S 0,2,1,4,1,5,1,6,2,7,1,9,1,5,4,11,1,12,1,13,5,5,1,17,2,5,4,16,1,20,1,

%T 18,4,6,6,24,1,5,5,24,1,24,1,18,11,5,1,30,1,15,3,18,1,30,6,30,5,7,1,

%U 39,1,3,18,31,6,34,1,16,3,34,1,44,1,4,13,16,4,39,1,42,5,5,1,48,5,5,2,41,1,51,2

%N Number of primes arising as the sum of one or more divisors of n.

%C a(2^n) = pi(2^(n+1)-1).

%C Except for n=3 and n=42, it appears that the records occur at the highly abundant numbers A002093. The record values appear to be pi(sigma(n)) for n in A002093, which means that these n are members of A093891. [_T. D. Noe_, Mar 19 2010]

%H T. D. Noe, <a href="/A093890/b093890.txt">Table of n, a(n) for n = 1..10000</a>

%e a(4) = 4, the divisors of 4 are 1, 2 and 4.

%e Primes arising are 2, 3 = 1 + 2, 5 = 1 + 4 and 7 = 1 + 2 + 4.

%t Do[l = Subsets[Divisors[n]]; l = Union[Map[Plus @@ #&, l]]; Print[Length[Select[l, PrimeQ]]], {n, 100}] (* _Ryan Propper_, Jun 04 2006 *)

%t CountPrimes[n_] := Module[{d=Divisors[n],t,lim,x}, t=CoefficientList[Product[1+x^i, {i,d}], x]; lim=PrimePi[Length[t]-1]; Count[t[[1+Prime[Range[lim]]]], _?(#>0 &)]]; Table[CountPrimes[n], {n,100}] (* _T. D. Noe_, Mar 19 2010 *)

%Y Cf. A093891, A093892.

%Y Cf. A161510 (primes counted with repetition). [_T. D. Noe_, Mar 19 2010]

%K nonn

%O 1,2

%A _Amarnath Murthy_, Apr 23 2004

%E Corrected and extended by _Ryan Propper_, Jun 04 2006

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Last modified March 28 04:13 EDT 2024. Contains 371235 sequences. (Running on oeis4.)