EDSRC:Critical sets in 3-space / by Matthew Grayson and Charles Pugh . Biquaternion algebras and quartic extensions / by Tsit Yuen Lam, David B. Leep, and Jean-Pierre Tignol . Projective varieties with non-residually finite fundamental group / by Domingo Toledo . Quantum invariants of links and 3-valent graphs in 3-manifolds / by Vladimir Turaev(Institut des hautes études scientifiques, 1993): p. 119 (Dept. of Math., Univ. of Utah, Salt Lake City)