# Good orientations of 2T-graphs

## TL;DR

This paper investigates conditions under which 2T-graphs admit acyclic orientations with arc-disjoint out- and in-branchings, providing constructive proofs and polynomial algorithms for such orientations.

## Contribution

It proves that all generic circuits have good orientations and characterizes 2T-graphs with specific inter-circuit edge structures that also admit good orientations.

## Key findings

- Every generic circuit has a good orientation.
- Certain 2T-graphs with structured inter-circuit edges have good orientations.
- The proofs are constructive, leading to polynomial algorithms for finding good orientations.

## Abstract

In this paper we study graphs which admit acyclic orientations that contain a pair of arc-disjoint out-branching and in-branching (such an orientation is called good) and we focus on edge-minimal such graphs. A 2T-graph is a graph whose edge set can be decomposed into two edge-disjoint spanning trees. Vertex-minimal 2T-graphs with at least two vertices which are known as generic circuits play an important role in rigidity theory for graphs. We prove that every generic circuit has a good orientation. Using this result we prove that if $G$ is 2T-graph whose vertex set has a partition $V_1,V_2,\ldots{},V_k$ so that each $V_i$ induces a generic circuit $G_i$ of $G$ and the set of edges between different $G_i$'s form a matching in $G$, then $G$ has a good orientation. We also obtain a characterization for the case when the set of edges between different $G_i$'s form a double tree, that is, if we contract each $G_i$ to one vertex, and delete parallel edges we obtain a tree. All our proofs are constructive and imply polynomial algorithms for finding the desired good orderings and the pairs of arc-disjoint branchings which certify that the orderings are good. We also identify a structure which can be used to certify a 2T-graph which does not have a good orientation.