  We study the unweighted Tree Augmentation Problem (TAP) via the Lasserre (Sum
of Squares) system. We prove an approximation guarantee of ($1.8+\epsilon$)
relative to an SDP relaxation, which matches the combinatorial approximation
guarantee of Even, Feldman, Kortsarz and Nutov in ACM TALG (2009), where
$\epsilon>0$ is a constant. We generalize the combinatorial analysis of
integral solutions of Even, et al., to fractional solutions by identifying some
properties of fractional solutions of the Lasserre system via the decomposition
result of Rothvo{\ss} (arXiv:1111.5473, 2011) and Karlin, Mathieu and Nguyen
(IPCO 2011).
