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A182910 Number of unitary prime divisors of the swinging factorial (A056040) n$ = n! / floor(n/2)!^2. 1
0, 0, 1, 2, 2, 3, 1, 2, 3, 3, 1, 2, 3, 4, 3, 3, 4, 5, 4, 5, 4, 6, 5, 6, 5, 5, 4, 4, 3, 4, 5, 6, 7, 8, 6, 6, 7, 8, 7, 7, 8, 9, 9, 10, 9, 7, 6, 7, 7, 7, 7, 8, 7, 8, 8, 10, 11, 13, 12, 13, 11, 12, 11, 10, 11, 13, 12, 13 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

A prime divisor of n is unitary iff its exponent is 1 in the prime power factorization of n. A unitary prime divisor of the swinging factorial n$ can be smaller than n/2. For n >= 30 the swinging factorial has more unitary prime divisors than the factorial and it never has fewer unitary prime divisors. Thus a(n) >= PrimePi(n) - PrimePi(n/2).

LINKS

Table of n, a(n) for n=0..67.

EXAMPLE

16$ = 2*3*3*5*11*13. So 16$ has one non-unitary prime divisor and a(16) = 4.

MAPLE

UnitaryPrimeDivisor := proc(f, n) local k, F; F := f(n):

add(`if`(igcd(iquo(F, k), k)=1, 1, 0), k=numtheory[factorset](F)) end;

A056040 := n -> n!/iquo(n, 2)!^2;

A182910 := n -> UnitaryPrimeDivisor(A056040, n);

seq(A182910(i), i=1..LEN);

MATHEMATICA

Table[Function[m, If[m == 1, 0, Count[FactorInteger[m][[All, -1]], 1]]][n!/Floor[n/2]!^2], {n, 0, 67}] (* Michael De Vlieger, Aug 02 2017 *)

PROG

(Python)

from sympy import factorint, factorial, floor

def a056169(n): return 0 if n==1 else sum([1 for i in factorint(n).values() if i==1])

def a056040(n): return factorial(n)/factorial(floor(n/2))**2

def a(n): return a056169(a056040(n))

print map(a, xrange(68)) # Indranil Ghosh, Aug 02 2017

CROSSREFS

Cf. A056171.

Sequence in context: A226743 A166269 A181648 * A055460 A067514 A115323

Adjacent sequences:  A182907 A182908 A182909 * A182911 A182912 A182913

KEYWORD

nonn

AUTHOR

Peter Luschny, Mar 14 2011

STATUS

approved

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Last modified October 18 03:13 EDT 2017. Contains 293486 sequences.