A011881 a(n) = floor(n*(n-1)/28).

0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 15, 16, 18, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 40, 42, 45, 47, 50, 52, 55, 58, 61, 64, 67, 70, 73, 77, 80, 84, 87, 91, 94, 98, 102, 106, 110, 114, 118

Offset

0,9

Links

Table of n, a(n) for n=0..58.

Index entries for linear recurrences with constant coefficients, signature (2, -1, 0, 0, 0, 0, 1, -2, 1, 0, 0, 0, 0, -1, 2, -1, 0, 0, 0, 0, 1, -2, 1).

Formula

G.f.: (x^6 (-1-(-1+x) x (1+(-1+x) x) (1+x^2) (1+x+x^2) (1-x^2+x^4)))/(-1+x (2+x (-1+(-1+x)^2 x^5 (1-x^7+x^14)))) Peter J. C. Moses, Jun 01 2014

a(n)= +2*a(n-1) -a(n-2) +a(n-7) -2*a(n-8) +a(n-9) -a(n-14) +2*a(n-15) -a(n-16) +a(n-21) -2*a(n-22) +a(n-23). - R. J. Mathar, Aug 11 2021

Mathematica

Table[Floor[(n(n-1))/28], {n, 0, 60}] (* or *) LinearRecurrence[{2, -1, 0, 0, 0, 0, 1, -2, 1, 0, 0, 0, 0, -1, 2, -1, 0, 0, 0, 0, 1, -2, 1}, {0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 15, 16}, 60] (* Harvey P. Dale, Apr 18 2022 *)

Crossrefs

Sequence in context: A062049 A025764 A237119 * A366853 A076678 A029024

Adjacent sequences: A011878 A011879 A011880 * A011882 A011883 A011884

Keyword

nonn,easy

Author

N. J. A. Sloane.

Status

approved