We prove a formula for Thom polynomials of A d singularities in any codimension. We use a combination of the test-curve model of Porteous, and the localization methods in equivariant cohomology. Our formulas are independent of the codimension, and are computationally effective up to d = 6.
IntroductionWe begin with a quick summary of the notions of global singularity theory and the theory of Thom polynomials. For a more detailed review we refer the reader to [1], [14].Consider a holomorphic map f : N → K between two complex manifolds, of dimensions n ≤ k. We say that p ∈ N is a singular point of f if the rank of the differential df p : T p N → T f (p) K is less than n.Topology often forces f to be singular at some points of N , and we will be interested in studying such situations. Before we proceed, we introduce a finer classification of singular points. Choose local coordinates near p ∈ N and f (p) ∈ K, and consider the resulting map-germf p : (C n , 0) → (C k , 0), which may be thought of as a sequence of k power series in n variables without constant terms. The group of infinitesimal local coordinate changes Diff(C k ) × Diff(C n ) acts on the space J (n, k) of all such map-germs. We will call Diff(C k ) × Diff(C n )-orbits or, more generally, Diff(C k ) × Diff(C n )-invariant subsets O ⊂ J (n, k) singularities. For a singularity O and holomorphic f : N → K, we can define the setwhich is independent of any coordinate choices. Then, under some additional technical assumptions (compact N , appropriately chosen closed O, and sufficiently generic f ),is an analytic subvariety of N . The computation of the Poincaré dual class α O [f ] ∈ H * (N, Z) of this set is one of the fundamental problems of global singularity theory. This is indeed useful: for example, A similar result, which we will call Thom's principle, has been used in the holomorphic category (cf. [14], [9] and §2 of the present paper). To formulate it in more concrete terms, denote by C[λ, θ] Sn×S k the space of those polynomials in the variables (λ 1 , . . . , λ n , θ 1 , . . . , θ k ) which are invariant under the permutations of the λs and the permutations of the θs. According to the structure theorem of symmetric polynomials, C[λ, θ] Sn×S k itself is a polynomial ring in the elementary symmetric polynomialsUsing the Chern-Weil map, a polynomial Q ∈ C[λ, θ] Sn×S k and a pair of bundles (E, F ) over N of ranks n and k, respectively, produces a characteristic class Q(E, F ) ∈ H * (N, C). Then the complex variant of Thom's principle reads:A precise version of this statement is described in Section 2. The polynomial Tp O is called the Thom polynomial of O, and the computation of these polynomials is a central problem of singularity theory.The structure of the Diff(C k ) × Diff(C n )-action on J (n, k) is rather complicated; even the parametrization of the orbits is difficult. There is, however, a simple invariant on the space of orbits: to each map-germf : (C n , 0) → (C k , 0), we can associate the finite-dimensional nilpotent algebra Af defin...