The weak order on the hyperoctahedral group and the monomial basis for the Hopf algebra of signed permutations

We give a combinatorial description for the weak order on the hyperoctahedral group. This characterization is then used to analyze the order-theoretic properties of the shifted products of hyperoctahedral groups. It is shown that each shifted product is a disjoint union of some intervals, which can be convex embedded into a hyperoctahedral group. As an application, we investigate the monomial basis for the Hopf algebra $\mathfrak{H}Sym$ of signed permutations, related to the fundamental basis via M\"obius inversion on the weak order on hyperoctahedral groups. It turns out that the image of a monomial basis element under the descent map from $\mathfrak{H}Sym$ to the algebra of type $B$ quasi-symmetric functions is either zero or a monomial quasi-symmetric function of type $B$.