#### by Rob Kirby

Joel Hass was born in Tel Aviv on 3 January, 1966, to Julian and Aliza Hass.

His mother was born and raised in Jerusalem to parents who had emigrated from Hungary (ca. 1919) to what would become Israel, then the British League of Nations Mandate for Palestine. Aliza’s father was a surveyor and her classmates included future Prime Minister Yitzhak Rabin.

Julian Hass was born in Waidhofen, Austria in 1919 and, shortly after, moved with his family to the town of Klagenfurt, where he grew up in poverty. In 1939, following the Nazi takeover of Austria, he escaped to Palestine. It was nearly impossible to obtain visas to travel, but a loophole allowed boats on the Danube, an international river requiring no visas, to take Jews to the Black Sea. There they transferred to ships which ran a British blockade to drop them off on the Israeli shore in the dark of night. After a brief stint on a kibbutz and working in construction, Julian joined the British army. He fought in Africa, including at the battle of El Alamein, and in the invasion of Italy. Julian was with the first group to reach Ferramonte, an Italian internment camp for Jews, where his sister Ida, with whom he had communicated by letter, had survived the war.

Julian eventually obtained the rank of sergeant and, at war’s end, returned to Israel to start a new civilian life. For a brief time, he was enrolled at Hebrew University, and there he met Aliza who was finishing a degree in chemistry and had spent the war driving ambulances in Egypt and Italy.

Julian’s time as a student was short-lived; after meeting an Italian textile manufacturer, he decided to go into business, and began running a thread manufacturer in Israel, and for a while prospered as an importer of textiles. When the enterprise folded, Julian and Aliza emigrated to New York. The year was 1960. Textiles was a dying industry in the US and Julian spent the remainder of his career working for a series of failing businesses.

Joel and his sister, Ada (who grew up to become a pediatrician) went to public schools in Queens, N.Y. They spent a year in London from 1967–68 and the years 1969–72 in Leicestershire, England. Joel did well in the local schools, which offered good teachers and labs. He did particularly well in math — it was easy for him — and his parents gave him a lot of encouragement in the subject.

When the family returned to New York, Joel did not enroll for his junior year of high school. Instead he entered Hunter College, one of the colleges in the City University of New York system, and a year later transferred to Columbia University. After graduating in 1976, he went to graduate school in math at the University of California, Berkeley.

At Berkeley he joined an illustrious group of topology students: Paul Melvin, John Hughes, Charles Livingston, Cole Giller, Bill Menasco, Tim Cochran, Daniel Ruberman, David Schorow, Bob Gompf (all Kirby students); Marc Culler (a student of John Stallings); Pat Gilmer and Bob Hasner (students of Emery Thomas); Bill Goldman (student of Moe Hirsch); and Roger Schlafly (student of Is Singer).

My memories of Joel during his grad student days revolve around games. We had a mixed soccer team in the intramurals, and Joel, Tim Cochran and Danny Ruberman were three of the stars. Joel was on a couple of memorable rafting trips, too, and was a good backgammon player. Mathematically, he had an eye for good problems, liked geometry as well as topology, and was an independent thinker, adept at finding his own research problems.

After finishing his PhD in 1981, Joel had a two-year postdoc appointment at the University of Michigan, which was deferred by one year so that he could participate in a special year in topology at Hebrew University. Then followed a year at MSRI in 1984–85, the famous year when the low-dimensional topology program was fortuitously joined by the operator algebras program with Vaughan Jones, in the wake of his work on the Jones polynomial (which had just been generalized to the HOMFLYPT two-variable polynomial).

During the spring of 1985 Joel met Abigail Thompson and they continued their courtship while Joel spent 1985–86 at Hebrew University. Together they spent 1986–87 at Hebrew University and were married in the summer of 1987. They spent the year 1987–88 at Berkeley, where their first daughter, Ellie, was born. They eventually settled at the University of California, Davis, where Benjamin Hass (1991) and Lucy Hass (1995) rounded out their family.

Shortly after arriving at Davis, both Joel and Abby learned to kayak, and both mastered the technique of rolling a kayak upright when it is flipped in a rapid. This led to many very enjoyable whitewater trips on California rivers, usually with me and the redoubtable Bruce Hammock, an excellent biologist.

As an established member of the math department at Davis, Joel spent 1990–91, 2000–2001 and 2015–16 at the Institute for Advanced Study; a semester at the University of Melbourne (2010), and semesters at the Technion (1992) and Jerusalem (2014). He was a successful chair of his department from 2010–14.

Some of Joel’s favorite papers are:

- The paper [1] that emerged from joint work with Mike Freedman and Peter Scott which explored the relationship between 3-dimensional manifolds and minimal surfaces. The paper showed that least area surfaces intersect as little as possible. Meeks and Yau had pioneered the application of minimal surfaces as a tool for studying 3-manifolds, applying their theory to minimal disks and spheres. This paper studied minimal incompressible surfaces of higher genus. It introduced a new notion of the complexity of two surfaces intersecting and extended the tower argument of Papakyriakopolous to arbitrary surfaces. The paper has become a standard tool in the study of 3-manifolds.
- The paper
[2]
constructed a metric on the 3-ball
__\( B^3 \)__with negative curvature and concave boundary. This is not possible with a hyperbolic metric, since a hyperbolic ball can be developed into__\( H^3 \)__. Bill Thurston had guessed that such a metric was not possible, giving a rare instance where his intuition did not point in the right direction. - Hass and Roger Schlafly solved the equal-volume double bubble
conjecture in
[4].
This paper was one of the earliest results
proved using rigorous computational methods, though this later
became quite common. It showed that a standard double bubble, formed
from two 2-spheres meeting at
__\( 120^o \)__and separated by a planar disk, gives the least area surface enclosing two regions of equal volume. - The construction of algorithms to solve problems in topology has
played an important role in topology since the work of
Dehn,
Haken
and
Markov.
A breakthrough in the further step of looking at the
complexity of a topological algorithm was given in
[3],
which
analyzed the complexity of algorithms for determining whether a knot
is trivial. Answering a question of Thurston, the paper
[3]
showed that
*unknotting*lies in the class NP. This paper helped pave the way for the current activity exploring a large number of topological complexity problems. - Since almost everything we see is a surface, surface comparison arises in numerous applications. The paper [5] covers this area, in which Joel has been working in recent years. He has been applying the powerful techniques developed in low dimensional topology and geometry to problems in shape recognition, in particular in biology and medicine. Much of radiology, for example, falls into the class of surface comparison problems. This paper describes a theory developed with Patrice Koehl that has been successfully used to compare and classify surfaces of objects such as bones, proteins and teeth.