4. Every network matrix is TU. The rows of a network matrix correspond to a tree , each of whose arcs has an arbitrary orientation (it is not necessary that there exist a root vertex "r" such that the tree is "rooted into "r"" or "out of "r"").The columns correspond to another set "C" of arcs on the same vertex set "V". To compute the entry at row "R" and column , look at the "s"-to-"t" path "P" in "T"; then the entry is: