OFFSET
0,1
COMMENTS
FORMULA
a(n) = 2*Product_{k=2..n} A004443(2*k).
a(n) = 2^(n-1)*(n+1)!/floor((n+1)/2), n > 0.
a(n) = 2^(n-1)*(1+(-1)^n)*((n-1)!+n!)-((-1)^n-1)*(2*n)!!/2, n > 0.
a(n) = 2*a(n-1)*(n+(-1)^n), n > 1, with a(1) = 2.
MATHEMATICA
a[n_] := Product[If[k == 1, 1, BitXor[2*k, 2]], {k, 0, n}]; Array[a, 21, 0] (* Amiram Eldar, Mar 19 2022 *)
PROG
(PARI) a(n) = 2*prod(k=2, n, bitxor(2*k, 2))
CROSSREFS
KEYWORD
nonn
AUTHOR
Peter McNair, Mar 19 2022
STATUS
approved