L. Sabalka, and D. Savchuk. (2010)cite arxiv:1007.1998
Comment: 18 pages, 5 figures. Revised version has many corrections and
revisions, including more explanation of definitions and proofs.
C. Olds, A. Lax, and G. Davidoff. Anneli Lax New Mathematical Library Mathematical Association of America, Washington, DC, (2000)Appendix I by Peter D. Lax.