Completely Independent Spanning Trees in $k$-Outerplanar Triangulated Discs
This study investigates the existence of two completely independent spanning trees (CISTs) in $k$-outerplanar triangulations. Through path-disjointness analysis, structural induction, and constructive proofs, it establishes for the first time that every 3-connected 2-outerplanar triangulation admits two CISTs. For the 3-outerplanar case, the work provides a sufficient condition guaranteeing the existence of such trees. Moreover, it constructs the first known counterexample—a 4-outerplanar triangulation that does not contain two CISTs. These results precisely delineate the boundary for CIST existence in low-level outerplanar graphs, significantly advancing the structural understanding of this important graph class.