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A116732 a(n) = a(n-1) + a(n-2) + a(n-3) - a(n-4). 5
0, 0, 0, 1, 1, 2, 4, 6, 11, 19, 32, 56, 96, 165, 285, 490, 844, 1454, 2503, 4311, 7424, 12784, 22016, 37913, 65289, 112434, 193620, 333430, 574195, 988811, 1702816, 2932392, 5049824, 8696221, 14975621, 25789274, 44411292, 76479966, 131704911, 226806895 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,6

COMMENTS

This sequence is an example of a "symmetric" quartic recurrence and has some expected divisibility properties.

a(n-3) counts partially ordered partitions of (n-3) into parts 1,2,3 where only the order of the adjacent 1's and 3's are unimportant (see example). - David Neil McGrath, Jul 25 2015

LINKS

Table of n, a(n) for n=0..39.

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

FORMULA

G.f.: x^3/(x^4 - x^3 - x^2 - x + 1).

EXAMPLE

Partially ordered partitions of (n-3) into parts 1,2,3 where only the order of adjacent 1's and 3's are unimportant. E.g., a(n-3)=a(6)=19. These are (33),(321),(312),(231),(123),(132),(3111),(2211),(1122),(1221),(2112),(2121),(1212),(21111),(12111),(11211),(11121),(11112),(111111). - David Neil McGrath, Jul 25 2015

MATHEMATICA

LinearRecurrence[{1, 1, 1, -1}, {0, 0, 0, 1}, 40] (* Vladimir Joseph Stephan Orlovsky, Feb 02 2012 *)

PROG

(PARI) v=[0, 0, 0, 1]; for(i=1, 40, v=concat(v, v[#v]+v[#v-1]+v[#v-2]-v[#v-3])); v \\ Derek Orr, Aug 27 2015

CROSSREFS

Close to A000786 (& A048239), A115992, A115993. Cf. A116201.

Sequence in context: A115992 A115993 A136424 * A048239 A000786 A000694

Adjacent sequences:  A116729 A116730 A116731 * A116733 A116734 A116735

KEYWORD

nonn,easy

AUTHOR

R. K. Guy, Mar 23 2008

EXTENSIONS

More terms from Max Alekseyev, Mar 23 2008

STATUS

approved

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Last modified December 6 03:18 EST 2016. Contains 278772 sequences.