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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A173125 Sum_{k=floor[n/2] mod 5} C(n,k). 2
1, 1, 2, 3, 6, 10, 20, 35, 70, 127, 254, 474, 948, 1807, 3614, 6995, 13990, 27370, 54740, 107883, 215766, 427351, 854702, 1698458, 3396916, 6765175, 13530350, 26985675, 53971350, 107746282, 215492564, 430470899, 860941798, 1720537327, 3441074654 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Greater of number of closed walks of length n from a node on a pentagon and number of walks of length n between two adjacent nodes on a pentagon.

LINKS

Table of n, a(n) for n=0..34.

Index entries for linear recurrences with constant coefficients, signature (2,3,-6,-1,2).

FORMULA

a(n) = A000045(n+1)+A173126(n). a(2n) = A054877(2n); a(2n+1) = A052964(2n).

a(n) = 2*a(n-1)+3*a(n-2)-6*a(n-3)-a(n-4)+2*a(n-5). - Colin Barker, Sep 14 2014

G.f.: -(x-1)*(x^3+3*x^2-1) / ((2*x-1)*(x^2-x-1)*(x^2+x-1)). - Colin Barker, Sep 14 2014

EXAMPLE

For n=15, k=7 mod 5 gives k=2, 7 or 12, and C(15,2)+C(15,7)+C(15,12) = 105+6435+455, so a(15)=6995.

PROG

(PARI) Vec(-(x-1)*(x^3+3*x^2-1)/((2*x-1)*(x^2-x-1)*(x^2+x-1)) + O(x^100)) \\ Colin Barker, Sep 14 2014

CROSSREFS

Sequence in context: A126930 A210736 A036557 * A047131 A231331 A008927

Adjacent sequences:  A173122 A173123 A173124 * A173126 A173127 A173128

KEYWORD

nonn,easy

AUTHOR

Henry Bottomley, Feb 10 2010

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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 19 22:28 EDT 2018. Contains 316378 sequences. (Running on oeis4.)