We determine the condition on a given lens space having a realization as a closure of homology cobordism over a planar surface with a given number of boundary components. As a corollary, we see that every lens space is represented as a closure of homology cobordism over a planar surface with three boundary components. In the proof of this corollary, we use Chebotarev density theorem.
Preliminary
Homology cobordisms (Section 2.4 of [2])A homology cobordism over Σ g,n (n ≥ 1) is a triad (X, ∂ + X, ∂ − X), where X is an oriented compact 3-manifold and ∂ + X ∪ ∂ − X is a partition of ∂X, and ∂ ± X are homeomorphic to Σ g,n satisfying: