OFFSET
0,2
FORMULA
a(n) = Sum_{k=0..2^(n-1)} (2^n)!/((2^n-2*k)! * k! * 2^k).
a(n) = A000085(2^n).
MATHEMATICA
Table[Sum[(2^n)!/((2^n - 2 k)!*k!*2^k), {k, 0, 2^(n - 1)}], {n, 0, 7}] (* Michael De Vlieger, Aug 04 2017 *)
PROG
(PARI) a(n) = sum(k=0, 2^(n-1), (2^n)!/((2^n-2*k)!*k!*2^k)) \\ Felix Fröhlich, Aug 04 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Sean A. Irvine, Aug 04 2017
STATUS
approved