In this paper, we introduce the concept of node expansion. Node expansion is a generalization of edge subdivision and an inverse of subgraph contraction. A graph \(G_1 = (V_1, E_1)\) is an \(H\)-node expansion of \(G = (V, E)\) if and only if \(G_1\) contains a subgraph \(H = (V_H, E_H)\) such that \(V = V_1 – V_H \cup \{v\}\) and \(E = E_1 – E_H \cup \{vw | wh \in E_1 \;\text{and}\; h \in V_H\} \cup \{v\}\). The concept of node expansion is of considerable importance in modernization of networks.

We consider the node expansion problem of transforming a graph to a bipartite graph with a minimum number of node expansions using \(K_2\). We show that the \(K_2\)-node expansion problem is NP-Complete for general graphs and remains so if the input graph has maximum degree 3. However, we present a \(O({n}^2 \log n + mn + p^3)\) algorithm for the case when the input graph is restricted to be planar \(3\)-connected and output graph is required to be planar bipartite.

An algorithm is presented for finding all \((0,1)\)-solutions to the matrix problem \(AX = J\), where \(A\) is a \((0,1)\)-matrix and \(J\) is the all \(1\)’s column vector. It is applied to the problem of enumerating distinct cyclic Steiner systems and five new values are obtained. Specifically, the number of distinct solutions to \(S(2,3,55), S(2,3,57), S(2,3,61), S(2,3,63)\), and \(S(3,4,22)\) are \(121,098,240, 84,672,512, 2,542,203,904, 1,782,918,144\), and \(1140\), respectively.

Let \(g_k(n) = \sum_{\underline{v}\in C_k(n)} \binom{n}{v} 2^{v_1v_2 + v_2v_3 + v_3v_4 + \ldots +v_{k-1}v_k}\) where \(C_k(n)\) denote the set of \(k\)-compositions of \(n\). We show that

1. \(g_k(n+p-1) \equiv g_k(n) \pmod{p}\) for all \(k,n \geq 1\), prime \(p\);
2. \(g_k(n)\) is a polynomial in \(k\) of degree \(n\) for \(k \geq n+1\);

and, moreover, that these properties hold for wider classes of functions which are sums involving multinomial coefficients.