Non-Orientable Lagrangian Cobordisms between Legendrian Knots

TL;DR

This paper demonstrates that all Legendrian knots admit non-orientable Lagrangian endocobordisms with specific topological constraints, contrasting with the orientable case, and explores their properties and implications in contact topology.

Contribution

It introduces the existence of non-orientable Lagrangian endocobordisms for all Legendrian knots and characterizes their topological features, unlike the orientable case.

Findings

Non-orientable Lagrangian endocobordisms exist for all Legendrian knots.

The crosscap genus of such cobordisms is a positive multiple of 4.

Exact, non-orientable Lagrangian endocobordisms exist for stabilized Legendrian knots but not for fillable ones.

Abstract

In the symplectization of standard contact $3$-space, $\mathbb R \times \mathbb R^3$, it is known that an orientable Lagrangian cobordism between a Legendrian knot and itself, also known as an orientable Lagrangian endocobordism for the Legendrian knot, must have genus $0$. We show that any Legendrian knot has a non-orientable Lagrangian endocobordism, and that the crosscap genus of such a non-orientable Lagrangian endocobordism must be a positive multiple of $4$. The more restrictive exact, non-orientable Lagrangian endocobordisms do not exist for any exactly fillable Legendrian knot but do exist for any stabilized Legendrian knot. Moreover, the relation defined by exact, non-orientable Lagrangian cobordism on the set of stabilized Legendrian knots is symmetric and defines an equivalence relation, a contrast to the non-symmetric relation defined by orientable Lagrangian cobordisms.