login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A124398 Denominators of partial sums of a series for sqrt(5)/3. 1
1, 5, 25, 25, 125, 3125, 15625, 78125, 78125, 390625, 9765625, 48828125, 244140625, 244140625, 48828125, 6103515625, 30517578125, 152587890625, 152587890625, 762939453125, 19073486328125, 95367431640625, 476837158203125 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Denominators of alternating sums over central binomial coefficients scaled by powers of 5.

Numerators are given by A124397.

For the rationals r(n) see the W. Lang link under A124397.

r(n) is not 1/3 times the rational sequence A123747/A123748 which converges to sqrt(5).

LINKS

Table of n, a(n) for n=0..22.

FORMULA

a(n)=denominator(r(n)) with the rationals r(n):=sum(((-1)^k)*binomial(2*k,k)/5^k,k=0..n) in lowest terms.

r(n)=sum(((-1)^k)*((2*k-1)!!/((2*k)!!)*(4/5)^k,k=0..n),n>=0, with the double factorials A001147 and A000165.

EXAMPLE

a(3)=25 because r(3)= 1-2/5+6/25-4/25 = 17/25 = A124397(3)/a(3).

CROSSREFS

Sequence in context: A265973 A265928 A039936 * A121003 A121007 A043057

Adjacent sequences:  A124395 A124396 A124397 * A124399 A124400 A124401

KEYWORD

nonn,frac,easy

AUTHOR

Wolfdieter Lang Nov 10 2006

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 July 19 08:23 EDT 2019. Contains 325155 sequences. (Running on oeis4.)