Demaine & Hajiaghayi (2004b). Demaine, Erik D.; Hajiaghayi, MohammadTaghi, Equivalence of local treewidth and linear local treewidth and its algorithmic applications, Proceedings of the Fifteenth Annual ACM-SIAM Symposium on Discrete Algorithms, New York: ACM: 840–849, 2004b, MR 2290974.
Kao (2008). Kao, Ming-Yang (编), Treewidth of graphs, Encyclopedia of Algorithms, Springer: 969, 2008, ISBN 9780387307701, Another long-standing open problem is whether there is a polynomial-time algorithm to compute the treewidth of planar graphs.