  In this expository paper we describe a powerful combinatorial formula and its
implications in geometry, topology, and algebra. This formula first appeared in
the appendix of a book by Andersen, Jantzen, and Soergel. Sara Billey
discovered it independently five years later, and it played a prominent role in
her work to evaluate certain polynomials closely related to Schubert
polynomials.
  Billey's formula relates many pieces of Schubert calculus: the geometry of
Schubert varieties, the action of the torus on the flag variety, combinatorial
data about permutations, the cohomology of the flag variety and of the Schubert
varieties, and the combinatorics of root systems (generalizing inversions of a
permutation). Combinatorially, Billey's formula describes an invariant of pairs
of elements of a Weyl group. On its face, this formula is a combination of
roots built from subwords of a fixed word. As we will see, it has deeper
geometric and topological meaning as well: (1) It tells us about the tangent
spaces at each permutation flag in each Schubert variety. (2) It tells us about
singular points in Schubert varieties. (3) It tells us about the values of
Kostant polynomials. Billey's formula also reflects an aspect of GKM theory,
which is a way of describing the torus-equivariant cohomology of a variety just
from information about the torus-fixed points in the variety.
  This paper will also describe some applications of Billey's formula,
including concrete combinatorial descriptions of Billey's formula in special
cases, and ways to bootstrap Billey's formula to describe the equivariant
cohomology of subvarieties of the flag variety to which GKM theory does not
apply.
