OFFSET
0,3
COMMENTS
The coefficients of the recursion for a(n) are given by the 3rd row of A145152.
Comment from - Enrique Navarrete, May 25 2020: (Start)
a(n-4) is the number of subsets of {1,2,...,n} such that the difference of successive elements is at least 4. For example, for n = 9, a(5) = 16 and the subsets are: {1,5}, {1,6}, {1,7}, {1,8}, {1,9}, {2,6}, {2,7}, {2,8}, {2,9}, {3,7}, {3,8}, {3,9}, {4,8}, {4,9}, {5,9}, {1,5,9}.
For n >=0 the sequence contains the triangular numbers; for n >= 4 have to add the tetrahedral numbers; for n >= 8 have to add the numbers binomial(n,4) (starting with 0,1,5,..); for n >= 12 have to add the numbers binomial(n,5) (starting with 0,1,6,..); in general, for n >= 4*k have to add to the sequence the numbers binomial(n, k+2), k >= 0.
For example, a(15) = 120+286+210+21, where 120 is a triangular number, 286 is a tetrahedral number, 210 is a number binomial(n,4) and 21 is a number binomial(m,5) (with the proper n, m due to shifts in the names of the sequences).
First difference is A098578.
(End)
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1,1,-2,1).
FORMULA
a(n) = 3a(n-1) -3a(n-2) +a(n-3) +a(n-4) -2a(n-5) +a(n-6).
EXAMPLE
a(7) = 38 = 3*25 -3*16 +10 +6 -2*3 +1.
MAPLE
col:= proc(k) local l, j, M, n; l:= `if` (k=0, [1, 0, 0, 1], [seq(coeff( -(1-x-x^4) *(1-x)^(k-1), x, j), j=1..k+3)]); M:= Matrix(nops(l), (i, j)-> if i=j-1 then 1 elif j=1 then l[i] else 0 fi); `if`(k=0, n->(M^n)[2, 3], n->(M^n)[1, 2]) end: a:= col(3): seq(a(n), n=0..40);
MATHEMATICA
Series[x/((1-x-x^4)*(1-x)^2), {x, 0, 50}] // CoefficientList[#, x]& (* Jean-François Alcover, Feb 13 2014 *)
LinearRecurrence[{3, -3, 1, 1, -2, 1}, {0, 1, 3, 6, 10, 16}, 50] (* Harvey P. Dale, Aug 08 2015 *)
PROG
(PARI) concat(0, Vec(1/(1-x-x^4)/(1-x)^2+O(x^99))) \\ Charles R Greathouse IV, Sep 25 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alois P. Heinz, Oct 03 2008
STATUS
approved