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

 

Logo

Thanks to everyone who made a donation during our annual appeal!
To see the list of donors, or make a donation, see the OEIS Foundation home page.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A191645 Numerators of the n-th partial "harmonic" sum of 1 + inverse semiprimes. 1
5, 17, 55, 293, 2141, 445, 457, 5153, 131597, 1745411, 1772711, 30586537, 31024117, 597115873, 604577173, 14050770329, 99311504603, 100230122303, 101081931443, 101903852543 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Denominators appear to be the same as A140123. The fractions begin: 5/4, 17/12, 55/36, 293/180, 2141/1260, 445/252, 457/252, 5153/2772, 131597/69300, ...

This is the semiprime analog of A024528.

LINKS

Table of n, a(n) for n=1..20.

EXAMPLE

a(1) =  5 because 1 + 1/4 = 5/4.

a(2) = 17 because 1 + 1/4 + 1/6 = 17/12.

a(3) = 55 because 1 + 1/4 + 1/6 + 1/9 = 55/36.

MAPLE

A191645 := proc(n) 1+add(1/A001358(i), i=1..n) ; numer(%) ; end proc:

seq(A191645(n), n=1..20) ; # R. J. Mathar, Jun 16 2011

MATHEMATICA

With[{sp=Join[{1}, Select[Range[100], PrimeOmega[#]==2&]]}, Rest[ Numerator[ Accumulate[1/sp]]]] (* Harvey P. Dale, May 01 2015 *)

PROG

(PARI) s=1; for(k=1, 99, bigomega(k)==2 & print1(numerator(s+=1/k)", "))  \\ M. F. Hasler, Jun 17 2011

CROSSREFS

Cf. A001358, A112141, A001008, A140123, A024528.

Sequence in context: A183712 A244235 A081495 * A146240 A112410 A146271

Adjacent sequences:  A191642 A191643 A191644 * A191646 A191647 A191648

KEYWORD

nonn,easy,frac

AUTHOR

Jonathan Vos Post, Jun 09 2011

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 January 21 11:11 EST 2020. Contains 331105 sequences. (Running on oeis4.)