Table of n, a(n) for n=1..7.
N. Graham and F. Harary, The number of perfect matchings in a hypercube, Appl. Math. Lett., 1 (1988), 45-48.
Patric R. J. Östergård and V. H. Pettersson, Enumerating Perfect Matchings in n-Cubes, Order, November 2013, Volume 30, Issue 3, pp 821-835.
Ville Pettersson, Graph Algorithms for Constructing and Enumerating Cycles and Related Structures, Preprint 2015.