login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A262095 Number of non-semiprime divisors of n. 1
1, 2, 2, 2, 2, 3, 2, 3, 2, 3, 2, 4, 2, 3, 3, 4, 2, 4, 2, 4, 3, 3, 2, 6, 2, 3, 3, 4, 2, 5, 2, 5, 3, 3, 3, 6, 2, 3, 3, 6, 2, 5, 2, 4, 4, 3, 2, 8, 2, 4, 3, 4, 2, 6, 3, 6, 3, 3, 2, 8, 2, 3, 4, 6, 3, 5, 2, 4, 3, 5, 2, 9, 2, 3, 4, 4, 3, 5, 2, 8, 4, 3, 2, 8, 3, 3, 3, 6, 2, 8, 3, 4, 3, 3, 3, 10, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A000005(n) - A086971(n).

A083399(n) <= a(n) <= A000005(n).

a(n) = Sum_{k=1..A000005(n)} (1 - A064911(A027750(n,k))). - Reinhard Zumkeller, Sep 14 2015

EXAMPLE

(1, 2, 3, 4, 6, 8, 12, 24) are the divisors of n = 24: 1, 2, 3, 8, 12, and 24 are non-semiprimes, therefore a(24) = 6.

MATHEMATICA

Table[Count[Divisors@ n, x_ /; PrimeOmega@ x != 2], {n, 97}] (* Michael De Vlieger, Sep 14 2015 *)

PROG

(PARI) a(n) = sumdiv(n, d, bigomega(d)!=2); \\ Michel Marcus, Sep 11 2015

(PARI) a(n)=my(f=factor(n)[, 2]); prod(i=1, #f, f[i]+1) - sum(i=1, #f, f[i]>1) - #f*(#f-1)/2 \\ Charles R Greathouse IV, Sep 14 2015

(Haskell)

a262095 = sum . map ((1 -) . a064911) . a027750_row

-- Reinhard Zumkeller, Sep 14 2015

CROSSREFS

Cf. A000005, A083399, A086971, A100959.

Cf. A064911, A027750.

Sequence in context: A081309 A329377 A010553 * A163374 A108502 A260235

Adjacent sequences:  A262092 A262093 A262094 * A262096 A262097 A262098

KEYWORD

nonn,easy

AUTHOR

Juri-Stepan Gerasimov, Sep 11 2015

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 27 15:29 EDT 2020. Contains 337383 sequences. (Running on oeis4.)