Before revision:  Despite lots of known results in the field of submodular maximization, surprisingly, it remains unknown whether or not this problem enjoys the well-known tight (1 - 1 / e )-approximation .
After revision:  Despite lots of known results in the field of submodular maximization, surprisingly, it remains unknown whether or not this problem enjoys the well-known tight (1 - 1 / e-\epsilon )-approximation .