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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A164870 The number of permutations of length n that can be sorted by 2 pop stacks in parallel. 2
1, 2, 6, 22, 84, 320, 1212, 4576, 17256, 65048, 245184, 924160, 3483408, 13129952, 49490592, 186544480, 703140672, 2650342784, 9989916864, 37654917376, 141932392320, 534984681344, 2016513669120, 7600829555200, 28649748728064, 107989278831104, 407043163037184 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

Anders Claesson, Mark Dukes and Martina Kubitzke, Partition and composition matrices, arXiv:1006.1312 [math.CO], 2010-2011.

R. Smith and V. Vatter, The enumeration of permutations sortable by pop stacks in parallel, Information Processing Letters, Volume 109, Issue 12, 31 May 2009, Pages 626-629.

Index entries for linear recurrences with constant coefficients, signature (6,-10,6)

FORMULA

G.f.: -x*(2*x-1)^2 / ( -1+6*x-10*x^2+6*x^3 ).

a(n) = 6*a(n-1) - 10*a(n-2) + 6*a(n-3) for n>3. - Colin Barker, Oct 31 2017

PROG

(PARI) Vec(x*(1 - 2*x)^2 / (1 - 6*x + 10*x^2 - 6*x^3) + O(x^30)) \\ Colin Barker, Oct 31 2017

CROSSREFS

Sequence in context: A150242 A150243 A200316 * A121686 A245904 A128723

Adjacent sequences:  A164867 A164868 A164869 * A164871 A164872 A164873

KEYWORD

nonn,easy

AUTHOR

Vincent Vatter, Aug 29 2009

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 21 21:57 EDT 2019. Contains 327282 sequences. (Running on oeis4.)