Ozsváth, Stipsicz and Szabó have defined a knot concordance invariant taking values in the group of piecewise linear functions on the closed interval . This paper presents a description of one approach to defining and proving its basic properties.
"Notes on the knot concordance invariant Upsilon." Algebr. Geom. Topol. 17 (1) 111 - 130, 2017. https://doi.org/10.2140/agt.2017.17.111