The On-Line Encyclopedia of Integer Sequences (OEIS)

Revision History for A079275 (Bold, blue-underlined text is an addition; faded, red-underlined text is a ~~deletion~~.)
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A079275

Number of divisors of n that are semiprimes with distinct factors.
(history; published version)

#25 by R. J. Mathar at Mon Jan 18 04:47:55 EST 2021

STATUS

editing

approved

#24 by R. J. Mathar at Mon Jan 18 04:47:25 EST 2021

MAPLE

A079275 := proc(n)

local a, d ;

a := 0 ;

for d in numtheory[divisors](n) do

if A001221(d) = 2 and A001222(d) = 2 then

a := a+1 ;

end if;

end do:

a ;

end proc:

seq(A079275(n), n=1..40) ; # R. J. Mathar, Jan 18 2021

STATUS

approved

editing

#23 by N. J. A. Sloane at Sat Sep 26 10:58:55 EDT 2020

STATUS

proposed

approved

#22 by David A. Corneth at Tue Sep 15 03:29:24 EDT 2020

STATUS

editing

proposed

#21 by David A. Corneth at Tue Sep 15 03:29:14 EDT 2020

PROG

(PARI) a(n) = binomial(omega(n), 2) \\ David A. Corneth, Sep 15 2020

STATUS

proposed

editing

#20 by Michel Marcus at Tue Sep 15 00:34:19 EDT 2020

STATUS

editing

proposed

#19 by Michel Marcus at Tue Sep 15 00:34:16 EDT 2020

PROG

(PARI) a(n) = sumdiv(n, d, (bigomega(d)==2) && (omega(d)==2)); \\ Michel Marcus, Sep 15 2020

STATUS

proposed

editing

#18 by Wesley Ivan Hurt at Mon Sep 14 23:31:55 EDT 2020

STATUS

editing

proposed

#17 by Wesley Ivan Hurt at Mon Sep 14 23:30:37 EDT 2020

COMMENTS

Number of pairs of prime factors of n, (p,q), such that p < q. For example, the prime factors of 30 are 2, 3 and 5, so we have the ordered pairs (2,3), (2,5) and (3,5). - Wesley Ivan Hurt, Sep 14 2020

FORMULA

a(n) = Sum_{p|n, q|n, p,q prime, p<q} 1. - Wesley Ivan Hurt, Sep 14 2020

KEYWORD

nonn,easy

STATUS

approved

editing

#16 by Michel Marcus at Sat May 06 03:47:46 EDT 2017

STATUS

reviewed

approved