Joel Hass at Berkeley in 1987 Joel Hass is an American mathematician and a professor of mathematics and at the University of California, Davis .^{[1]} His work focuses on geometric and topological problems in dimension 3.

Biography
Hass received his Ph.D. from the University of California, Berkeley in 1981 under the supervision of Robion Kirby .^{[2]} He joined the Davis faculty in 1988.^{[1]}

In 2012 he became a fellow of the American Mathematical Society .^{[3]} From 2010 to 2014 he served as the chair of the UC Davis mathematics department.^{[4]}

Research contributions
Hass is known for proving the equal-volume special case of the double bubble conjecture , for proving that the unknotting problem is in NP , and for giving an exponential bound on the number of Reidemeister moves needed to reduce the unknot to a circle.

Selected publications
Research papers Freedman, Michael ; Hass, Joel; Scott, Peter (1983), "Least area incompressible surfaces in 3-manifolds" (PDF) , Inventiones Mathematicae , 71 (3): 609–642, Bibcode :1983InMat..71..609F , doi :10.1007/BF02095997 , hdl :2027.42/46610 , MR 0695910 , S2CID 42502819 .
Hass, Joel; Lagarias, Jeffrey C. ; Pippenger, Nicholas (1999), "The computational complexity of knot and link problems", Journal of the ACM , 46 (2): 185–211, arXiv :math/9807016 , doi :10.1145/301970.301971 , S2CID 125854 .
Hass, Joel; Schlafly, Roger (2000), "Double bubbles minimize", Annals of Mathematics , Second Series, 151 (2): 459–515, arXiv :math/0003157 , Bibcode :2000math......3157H , doi :10.2307/121042 , JSTOR 121042 , MR 1765704 , S2CID 15663910 .
Hass, Joel; Lagarias, Jeffrey C. (2001), "The number of Reidemeister moves needed for unknotting", Journal of the American Mathematical Society , 14 (2): 399–428, arXiv :math/9807012 , doi :10.1090/S0894-0347-01-00358-7 , MR 1815217 , S2CID 15654705 .Books Adams, Colin ; Hass, Joel; Thompson, Abigail (1998), How to Ace Calculus: The Streetwise Guide , New York: W.H. Freeman and Company, ISBN 0-7167-3160-6 .
Adams, Colin ; Hass, Joel; Thompson, Abigail (2001), How to Ace the Rest of Calculus: The Streetwise Guide , New York: W.H. Freeman and Company, ISBN 0-7167-4174-1 .2004: Student Solutions Manual, Maurice D. Weir, Joel Hass, George B. Thomas, Frank R Giordano