Abstract
We demonstrate that ℓ rounds of the Sherali-Adams hierarchy and 2ℓ rounds of the Lovász-Schrijver hierarchy suffice to reduce the integrality gap of a natural LP relaxation for Directed Steiner Tree in ℓ-layered graphs from $\Omega(\sqrt k)$ to O(ℓ·logk) where k is the number of terminals. This is an improvement over Rothvoss’ result that 2ℓ rounds of the considerably stronger Lasserre SDP hierarchy reduce the integrality gap of a similar formulation to O(ℓ·logk). We also observe that Directed Steiner Tree instances with 3 layers of edges have only an O(logk) integrality gap in the standard LP relaxation, complementing the known fact that the gap can be as large as $\Omega(\sqrt k)$ in graphs with 4 layers.
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