Large induced subgraph with a given pathwidth in outerplanar graphs

A long-standing conjecture by Albertson and Berman states that every planar graph of order $n$ has an induced forest with at least $\lceil \frac{n}{2} \rceil$ vertices. As a variant of this conjecture, Chappell conjectured that every planar graph of order $n$ has an induced linear forest with at least $\lceil \frac{4n}{9} \rceil$ vertices. Pelsmajer proved that every outerplanar graph of order $n$ has an induced linear forest with at least $\lceil \frac{4n+2}{7}\rceil$ vertices and this bound is sharp. In this paper, we investigate the order of induced subgraphs of outerplanar graphs with a given pathwidth. The above result by Pelsmajer implies that every outerplanar graph of order $n$ has an induced subgraph with pathwidth one and at least $\lceil \frac{4n+2}{7}\rceil$ vertices. We extend this to obtain a result on the maximum order of any outerplanar graph with at most a given pathwidth. We also give its upper bound which generalizes Pelsmajer's construction.