On Factorable Bigraphic Pairs

Let $S = (a_1,. . ., a_m; b_1, . . ., b_n)$, where $a_1, . . ., a_m$ and $b_1, . . ., b_n$ are two sequences of nonnegative integers. We say that $S$ is a bigraphic pair if there exists a simple bipartite graph $G$ with partite sets ${x_1, x_2, . . ., x_m}$ and ${y_1, y_2, . . ., y_n}$ such that $d_G(x_i) = a_i$ for $1 ≤ i ≤ m$ and $d_G(y_j) = b_j$ for $1 ≤ j ≤ n$. In this case, we say that $G$ is a realization of $S$. Analogous to Kundu’s $k$-factor theorem, we show that if $(a_1, a_2, . . ., a_m; b_1, b_2, . . ., b_n)$ and $(a_1 − e_1, a_2 − e_2, . . ., a_m − e_m; b_1 − f_1, b_2 − f_2, . . ., b_n − f_n)$ are two bigraphic pairs satisfying $k ≤ f_i ≤ k + 1, 1 ≤ i ≤ n$ (or$ k ≤ e_i ≤ k + 1, 1 ≤ i ≤ m$), for some $0 ≤ k ≤ m − 1$ (or $0 ≤ k ≤ n − 1$), then $(a_1, a_2, . . ., a_m; b_1, b_2, . . ., b_n)$ has a realization containing an $(e_1, e_2, . . ., e_m; f_1, f_2, . . ., f_n)$-factor. For $m = n$, we also give a necessary and sufficient condition for an $(k^n; k^n)$-factorable bigraphic pair to be connected $(k^n; k^n)$-factorable when $k ≥ 2$. This implies a characterization of bigraphic pairs with a realization containing a Hamiltonian cycle.