# Minimum cost zero skew tree

Let T be a rooted tree with real costs c(e) and length lower bounds \ell(e) on each edge e. We are interested in compute a function f, such that f(e)\geq \ell(e), for any root to leaf path P, \sum_{e\in P} f(e)=t and \sum_{e\in E} c(e)f(e) is minimized.

This is the zero skew tree problem where the tree is fixed.

Here we show how this problem can be solved in O(n\log n) time by reducing it to minimum cost flow on 2-terminal series parallel graph. [1]

C(v) denote the set of all the children of v.

For vu\in E(T), and u\in C(v), the graph P(vu) is the parallel connection of one single edge with cost c(vu), lower bound \ell(vu), infinite upper bound and a series-parallel graph S(u). Let the graph S(v) for v\in V(T) to be the series connection of P(vu) for all u\in C(v)(the connection can be ordered arbitrarily).

The base case is when u does not have any children, and P(vu) is just a single edge with two terminals.

Let G=S(r), where r is the root of T. There is a bijection between the edges in T and the edges in G.

Finding the minimum cost flow of value t in G with the two terminals gives us the desired solution by going back to the original edge using the bijection.

# References

[1] H. Booth, R. Tarjan, **Finding the minimum-cost maximum flow in a series-parallel network**, Journal of Algorithms. 15 (1993) 416–446 10.1006/jagm.1993.1048.