Why is $tb(K)$ (Thurston-Bennequin invariant) of a Legendrian knot $K$ which is the boundary of a convex surface $\Sigma$ is negative in a contact 3 manifold?
Why is $tb(K)$ (Thurston-Bennequin invariant) of a knot $K$ which is the boundary of a convex surface $\Sigma$ is negative in a contact 3 manifold?