APUD(1,1) Recognition in Polynomial Time

TL;DR

This paper proves that recognizing APUD(1,1) graphs, a special case of unit disk graphs constrained to two lines, can be done efficiently in polynomial time, contrasting with the NP-hardness of the general problem.

Contribution

The paper demonstrates that APUD(1,1) recognition is solvable in polynomial time, providing a significant complexity classification for this specific graph recognition problem.

Findings

APUD(1,1) recognition is polynomial time solvable.

General APUD(k,m) recognition is NP-hard.

Contrasts with NP-hardness of broader cases.

Abstract

A unit disk graph is the intersection graph of a set of disk of unit radius in the Euclidean plane. In 1998, Breu and Kirkpatrick showed that the recognition problem for unit disk graphs is NP-hard. Given $k$ horizontal and $m$ vertical lines, an APUD($k,m$) is a unit disk graph such that each unit disk is centered either on a given horizontal or vertical line. \c{C}a\u{g}{\i}r{\i}c{\i} showed in 2020 that APUD($k,m$) recognition is NP-hard. In this paper, we show that APUD($1,1$) recognition is polynomial time solvable.