Passage: Joan Sylvia Lyttle Birman (born May 30, 1927 in New York City) is an American mathematician, specialising in low-dimensional topology. She has given contributions to the study of knots, 3-manifolds, mapping class groups of surfaces, geometric group theory, contact structures and dynamical systems. Birman is currently Research Professor Emerita at Barnard College, Columbia University, where she has been since 1973.

Question: What field of math was Joan Birman's expertise?
low-dimensional topology