OFFSET
1,3
FORMULA
a(n) = n!! - (n-1)!! + (n-2)!! - ... 1!! = A006882(n) - a(n-1).
EXAMPLE
a(5) = 5!! - 4!! + 3!! - 2!! + 1!! = 15 - 8 + 3 - 2 + 1 = 9.
MAPLE
A129831 := proc(n)
add( (-1)^i*doublefactorial(n-i), i=0..n-1) ;
end proc: # R. J. Mathar, Aug 22 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
Paolo P. Lava & Giorgio Balzarotti, May 21 2007
STATUS
approved