Diagonal Knots and the Tau Invariant

In 2003, Ozsváth, Szabó and Rasmussen introduced the τ invariant for knots, and in 2011, Sarkar [‘Grid diagrams and the Ozsváth–Szabó tau-invariant’, Math. Res. Lett. 18 (6) (2011), 1239–1257] published a computational shortcut for the τ invariant of knots that can be represented by diagonal grid diagrams. Previously, the only knots known to have diagonal grid diagram representations were torus knots. We prove that all such knots are positive knots and we produce an example of a knot with a diagonal grid diagram representation which is not a torus knot.