A205745 -id:A205745 - cp's OEIS frontend

A305614 Expansion of Sum_{p prime} x^p/(1 + x^p).

0, 0, 1, 1, -1, 1, 0, 1, -1, 1, 0, 1, -2, 1, 0, 2, -1, 1, 0, 1, -2, 2, 0, 1, -2, 1, 0, 1, -2, 1, -1, 1, -1, 2, 0, 2, -2, 1, 0, 2, -2, 1, -1, 1, -2, 2, 0, 1, -2, 1, 0, 2, -2, 1, 0, 2, -2, 2, 0, 1, -3, 1, 0, 2, -1, 2, -1, 1, -2, 2, -1, 1, -2, 1, 0, 2, -2, 2, -1

Offset: 0

Gus Wiseman, Jun 06 2018

sign

a(n) is the number of prime divisors p|n such that n/p is odd, minus the number of prime divisors p|n such that n/p is even.

			The prime divisors of 12 are 2, 3. We see that 12/2 = 6, 12/3 = 4. None of those are odd, but both of them are even, so a(12) = -2.
The prime divisors of 30 are {2,3,5} with quotients {15,10,6}. One of these is odd and two are even, so a(30) = 1 - 2 = -1.

Robert Israel, Table of n, a(n) for n = 0..10000

Cf. A000005, A000607, A001221, A008683, A048165, A083399, A088705, A106404, A205745, A280954.

a:= n-> -add((-1)^(n/i[1]), i=ifactors(n)[2]):
seq(a(n), n=0..100);  # Alois P. Heinz, Jun 07 2018
# Alternative
N:= 1000: # to get a(0)..a(N)
V:= Vector(N):
p:= 1:
do
  p:= nextprime(p);
  if p > N then break fi;
  R:= [seq(i,i=p..N,p)];
  W:= ;
  V[R]:= V[R]+W;
od:
[0,seq(V[i],i=1..N)]; # Robert Israel, Jun 07 2018

Table[Sum[If[PrimeQ[d], (-1)^(n/d - 1), 0], {d, Divisors[n]}], {n, 30}]

a(n) = -Sum_{p|n prime} (-1)^(n/p).
From Robert Israel, Jun 07 2018: (Start)
If n is odd, a(n) = A001221(n).
If n == 2 (mod 4), a(n) = 2 - A001221(n).
If n == 0 (mod 4) and n > 0, a(n) = -A001221(n). (End)
L.g.f.: log(Product_{k>=1} (1 + x^prime(k))^(1/prime(k))) = Sum_{n>=1} a(n)*x^n/n. - Ilya Gutkovskiy, Jul 30 2018

A106404 Number of even semiprimes dividing n.

Original entry on oeis.org

0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 2, 0, 1, 0, 1, 0, 1, 0, 2, 0, 1, 0, 2, 0, 1, 0, 2, 0, 2, 0, 1, 0, 1, 0, 2, 0, 1, 0, 2, 0, 2, 0, 2, 0, 1, 0, 2, 0, 1, 0, 2, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 1, 0, 2, 0, 2, 0, 2, 0, 2, 0, 1, 0, 2, 0, 2, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 2, 0, 2, 0, 1, 0, 2, 0, 1, 0, 2, 0, 2, 0, 2, 0

Offset: 1

Reinhard Zumkeller, May 02 2005

nonn

Also the number of prime divisors p|n such that n/p is even. - Gus Wiseman, Jun 06 2018

			a(60) = #{4, 6, 10} = #{2*2, 2*3, 2*5} = 3.

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Cf. A000005, A000607, A001221, A001227, A008683, A083399, A088705, A100484, A205745, A305614.

a106404 n = length [d | d <- takeWhile (<= n) a100484_list, mod n d == 0]
-- Reinhard Zumkeller, Jan 31 2012

Table[Length[Select[Divisors[n],PrimeQ[#]&&EvenQ[n/#]&]],{n,100}] (* Gus Wiseman, Jun 06 2018 *)
Table[Count[Divisors[n],?(EvenQ[#]&&PrimeOmega[#]==2&)],{n,110}] (* _Harvey P. Dale, May 04 2021 *)
a[n_] := If[EvenQ[n], PrimeNu[n/2], 0]; Array[a, 100] (* Amiram Eldar, Jun 26 2022 *)

a(n)=if(n%2,0,omega(n/2)) \\ Charles R Greathouse IV, Jan 30 2012

def A106404(n):
    return add(1-(n/d)%2 for d in divisors(n) if is_prime(d))
print([A106404(n) for n in (1..105)])  # Peter Luschny, Jan 30 2012

a(n) = A086971(n) - A106405(n).
a(A100484(n)) = 1.
a(A005408(n)) = 0.
a(A005843(n)) > 0 for n>1.
a(2n) = omega(n), a(2n+1) = 0, where omega(n) is the number of distinct prime divisors of n, A001221. - Franklin T. Adams-Watters, Jun 09 2006
a(n) = card { d | d*p = n, d even, p prime }. - Peter Luschny, Jan 30 2012
O.g.f.: Sum_{p prime} x^(2p)/(1 - x^(2p)). - Gus Wiseman, Jun 06 2018

A347957 Dirichlet convolution of A001221 (omega) with A003602 (Kimberling's paraphrases).

Original entry on oeis.org

0, 1, 1, 2, 1, 5, 1, 3, 3, 6, 1, 9, 1, 7, 7, 4, 1, 14, 1, 11, 8, 9, 1, 13, 4, 10, 8, 13, 1, 28, 1, 5, 10, 12, 9, 25, 1, 13, 11, 16, 1, 34, 1, 17, 22, 15, 1, 17, 5, 25, 13, 19, 1, 38, 11, 19, 14, 18, 1, 49, 1, 19, 26, 6, 12, 46, 1, 23, 16, 44, 1, 36, 1, 22, 31, 25, 12, 52, 1, 21, 22, 24, 1, 60, 14, 25, 19, 25, 1, 86

Offset: 1

Antti Karttunen, Sep 20 2021

nonn

Antti Karttunen, Table of n, a(n) for n = 1..16384
Jon Maiga, Computer-generated formulas for A347957, Sequence Machine.

Cf. A001221, A003602, A023900, A181988, A205745.

Cf. also A328203, A347954, A347955, A347956.

A003602(n) = (1+(n>>valuation(n,2)))/2;
A347957(n) = sumdiv(n,d,omega(n/d)*A003602(d));

a(n) = Sum_{d|n} A001221(n/d) * A003602(d).
From Antti Karttunen, Nov 13 2021: (Start)
The following two convolutions were found by Jon Maiga's Sequence Machine search algorithm. The first one is obvious, and even the second one should not be too hard to prove:
a(n) = Sum_{d|n} A023900(n/d) * A347956(d).
a(n) = Sum_{d|n} A181988(n/d) * A205745(d).
(End)

A068050 Number of values of k, 1<=k<=n, for which floor(n/k) is prime.

Original entry on oeis.org

0, 1, 1, 1, 2, 2, 3, 2, 2, 4, 5, 3, 4, 5, 6, 5, 6, 5, 6, 6, 7, 9, 10, 6, 7, 9, 9, 9, 10, 10, 11, 9, 10, 12, 14, 11, 12, 13, 14, 13, 14, 13, 14, 14, 15, 17, 18, 13, 14, 16, 17, 18, 19, 17, 19, 18, 19, 21, 22, 18, 19, 20, 21, 19, 21, 22, 23, 23, 24, 26, 27, 21, 22, 23, 24, 24, 26, 27

Offset: 1

Amarnath Murthy, Feb 12 2002

			a(10) = 4 as floor(10/k) for k = 1 to 10 is 10,5,3,2,2,1,1,1,1,1, respectively; this is prime for k = 2,3,4,5.

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Randell Heyman, Primes in floor function sets, arXiv:2111.00408 [math.NT], 2021.

Cf. A067514.

Cf. A010051, A179119, A205745.

a068050 n = length [k | k <- [1..n], a010051 (n `div` k) == 1]
-- Reinhard Zumkeller, Jan 31 2012

a[n_] := Length[Select[Table[Floor[n/i], {i, 1, n}], PrimeQ]]
Table[Count[Table[Floor[n/k],{k,n}],?PrimeQ],{n,80}] (* _Harvey P. Dale, Nov 19 2022 *)

a(n) = sum(k=1, n, isprime(n\k)); \\ Michel Marcus, Jun 03 2024

If p is a prime other than 3, a(p) = a(p-1) + 1. - Franklin T. Adams-Watters, Apr 27 2020
a(n) = A179119*n + O(n^(1/2)). - Randell Heyman, Oct 06 2022
a(n) = Sum_{p prime and p<=n} (floor(n/p) - floor(n/(p+1))). - Ridouane Oudra, Jun 03 2024

Edited by Dean Hickerson, Feb 12 2002

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A305614 Expansion of Sum_{p prime} x^p/(1 + x^p).

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A106404 Number of even semiprimes dividing n.

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A347957 Dirichlet convolution of A001221 (omega) with A003602 (Kimberling's paraphrases).

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A068050 Number of values of k, 1<=k<=n, for which floor(n/k) is prime.

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