A simple classification of the special matchings in the symmetric group

Abstract: We establish a simple characterization of special matchings of all Bruhat intervals in the symmetric group, yielding an explicit classification that depends only on the one-line notations of the endpoints. We also study extensions of graded posets by partial special matchings, proving the existence and uniqueness of such extensions and providing an explicit construction. Combining these results, we obtain a combinatorial algorithm for computing Kazhdan--Lusztig polynomials for an isomorphism-closed family of Bruhat intervals that includes all intervals whose bottom element has length at most one.