K2 factors of Koszul algebras and applications to face rings

Generalizing the notion of a Koszul algebra, a graded k-algebra A is K2 if its Yoneda algebra ExtA(k,k) is generated as an algebra in cohomology degrees 1 and 2. We prove a strong theorem about K2 factor algebras of Koszul algebras and use that theorem to show the Stanley–Reisner face ring of a simplicial complex Δ is K2 whenever the Alexander dual simplicial complex Δ⁎ is (sequentially) Cohen–Macaulay.