The tree of decomposition of a biconnected graph

Abstract: The tree of decomposition of a $k$-connected graph by a set $\mathfrak S$ of pairwise independent $k$-vertex cutsets is defined as follows. The vertices of this tree are cutsets of $\mathfrak S$ and parts of decomposition of the graph by the set $\mathfrak S$, each cutset is adjacent to all parts that contain it. We prove, that the graph described above is a tree. The tree of decomposition of a biconnected graph is a particular case of this construction: it is the tree of decomposition of a biconnected graph by the set of all its single cutsets (i.e. 2-vertex cutsets, that are independent with all other 2-vertex cutsets). We show that this tree has much in common with the classic tree of blocks and cutpoints of a connected graph. With the help of the tree of decomposition of a biconnected graph we prove a planarity criterium and find some upper bounds on the chromatic number of this graph. Finally, we study the structure of critical biconnected graphs and prove that each such graph has at least four vertices of degree 2.