Generating families and augmentations for Legendrian surfaces

Abstract

We study augmentations of a Legendrian surface $L$ in the $1$–jet space, $J^{1}M$, of a surface $M$. We introduce two types of algebraic/combinatorial structures related to the front projection of $L$ that we call chain homotopy diagrams (CHDs) and Morse complex $2$–families (MC2Fs), and show that the existence of a $\rho$–graded CHD or a $\rho$–graded MC2F is equivalent to the existence of a $\rho$–graded augmentation of the Legendrian contact homology DGA to $\mathbb{Z}∕2$. A CHD is an assignment of chain complexes, chain maps, and homotopy operators to the $0$–, $1$–, and $2$–cells of a compatible polygonal decomposition of the base projection of $L$ with restrictions arising from the front projection of $L$. An MC2F consists of a collection of formal handleslide sets and chain complexes, subject to axioms based on the behavior of Morse complexes in $2$–parameter families. We prove that if a Legendrian surface has a tame-at-infinity generating family, then it has a $0$–graded MC2F and hence a $0$–graded augmentation. In addition, continuation maps and a monodromy representation of ${\pi }_1\left(M\right)$ are associated to augmentations, and then used to provide more refined obstructions to the existence of generating families that (i) are linear at infinity or (ii) have trivial bundle domain. We apply our methods in several examples.