|
|
A293042
|
|
Number of even permutations p of {1,...,n} such that p(i) is not i or i+1.
|
|
1
|
|
|
1, 0, 0, 1, 1, 9, 47, 339, 2705, 24402, 244294, 2689669, 32297985, 420096147, 5883813373, 88287031271, 1412982765793, 24026200566404, 432554403678604, 8219863859175945, 164419973194802817, 3453229295483253853, 75978854506098365995, 1747670263607990439483
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,6
|
|
LINKS
|
|
|
FORMULA
|
a(n) = (A000271(n) + (-1)^(n-1) * floor((n-1)/2))/2.
|
|
MATHEMATICA
|
a[n_] := (Sum[(-1)^(n-k) k! Binomial[n+k, 2 k], {k, 0, n}] + (-1)^(n-1)* Floor[(n-1)/2])/2; Table[a[n], {n, 0, 23}] (* Jean-François Alcover, Feb 18 2019 *)
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|