%N Number of nodes in the BDD for the hidden weighted bit function h_n under the best possible ordering of variables.
%C In this problem we don't consider "complement bits" to shorten the BDD.
%C The best method presently known to find a(n) takes something like 2.5^n steps.
%D Beate Bollig, Martin Löbbing, Martin Sauerhoff and Ingo Werner, On the complexity of the hidden weighted bit function for various BDD models, Theoretical Informatics and Applications, 33 (1999), 103-115, Theorem 4.4.
%D Randal E. Bryant, "On the complexity of VLSI implementations and graph representations of Boolean functions with application to integer multiplication," IEEE Transactions on Computers C-40 (1991), 205-213.
%D D. E. Knuth, The Art of Computer Programming, Vol. 4A, Section 7.1.4.
%e For example, when n=8 the smallest BDD is obtained when one tests first x8 (1 node), then x7 (2 nodes), then x1 (4), then x6 (6), then x2 (9), then x5 (12), then x4 (8), then x3 (2). The total number of nodes is 46, including the two sink nodes.
%Y Cf. A136445.
%A _Don Knuth_, Apr 23 2008