OFFSET
0,2
COMMENTS
A signed permutation is a sequence (x_1,x_2,...,x_n) of integers such that {|x_1|,|x_2|,...|x_n|} = {1,2...,n}.
Adjacent elements that differ in sign will always differ by more than 1.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..200
FORMULA
EXAMPLE
In the following examples, the number of assignments of signs to each unsigned permutation is shown in parenthesis.
a(2) = 4: 12(2), 21(2). Total is 2 + 2 = 4.
a(3) = 20: 123(2), 132(4), 213(4), 231(4), 312(4), 321(2).
PROG
(PARI) a(n)=subst(serlaplace(polcoef((1 + x)/(1 + (1 - 2*y)*x + 2*y*x^2) + O(x*x^n), n)), y, 1)
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Mar 01 2024
STATUS
approved