login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A253470 Indices of centered triangular numbers (A005448) which are also centered pentagonal numbers (A005891). 2
1, 5, 36, 280, 2201, 17325, 136396, 1073840, 8454321, 66560725, 524031476, 4125691080, 32481497161, 255726286205, 2013328792476, 15850904053600, 124793903636321, 982500325036965, 7735208696659396, 60899169248238200, 479458145289246201, 3774765993065731405 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also indices of pentagonal numbers (A000326) which are also centered pentagonal numbers (A005891).
Also positive integers x in the solutions to 3*x^2 - 5*y^2 - 3*x + 5*y = 0, the corresponding values of y being A182432.
LINKS
FORMULA
a(n) = 9*a(n-1)-9*a(n-2)+a(n-3).
G.f.: x*(4*x-1) / ((x-1)*(x^2-8*x+1)).
a(n) = (6-(4-sqrt(15))^n*(3+sqrt(15))+(-3+sqrt(15))*(4+sqrt(15))^n)/12. - Colin Barker, Mar 03 2016
EXAMPLE
5 is in the sequence because the 5th centered triangular number is 31, which is also the 4th centered pentagonal number.
PROG
(PARI) Vec(x*(4*x-1)/((x-1)*(x^2-8*x+1)) + O(x^100))
CROSSREFS
Sequence in context: A285392 A327091 A201351 * A188899 A052203 A332624
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jan 01 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 23 09:42 EST 2024. Contains 370276 sequences. (Running on oeis4.)