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A logarithmic connection determines meridian eigenvalues from its residue

Statement

Let E be a finite-dimensional complex vector space, let ζ=eiθ be a root of unity with 0<θ≤2π the positive arc angle (taking θ=2π when ζ=1), and let A(z) be a holomorphic family of endomorphisms on a disk about 0 with A(0)=R. Consider the logarithmic connection ∇=d−A(z)dzz on the punctured disk. Suppose the endpoint identification g:Eζr→Er is a fixed automorphism of the model fiber and satisfies gA(ζz)g−1=A(z). At z=0 this implies gR=Rg. Parallel transport from r to ζr along the positive arc, followed by this endpoint identification g, has characteristic polynomial equal to that of geiθR.

Facts & Assumptions

Given: E, ζ=eiθ, the holomorphic family A(z), its residue R=A(0), and the compatible endpoint identification g.

[F1]

A horizontal section for ∇=d−A(z) dz/z along z(t)=reit satisfies the finite-dimensional ODE u˙(t)=iA(reit)u(t).

[F2]

The coefficient identity gA(ζz)g−1=A(z) expresses equivariance under z↦ζz with the stated endpoint identification from the endpoint fiber to the initial fiber.

[F3]

Linear matrix ODEs have unique global solutions on a fixed interval gives existence and uniqueness of transport along compact paths, interpreting complex matrices as real linear operators. Gronwall's integral inequality with variable and constant coefficients gives the uniform convergence estimate below.

Proof

technique · direct

Given: [F1]–[F3].

1.1F2F3givenalgebra

For r>0, let Pr be parallel transport along the arc z(t)=reit, 0≤t≤θ, and put Mr:=gPr, an endomorphism of the initial fiber. The connection form Ω=A(z) dz/z is flat on the punctured disk: holomorphicity gives dΩ=0, and Ω∧Ω=0 because both factors are multiples of dz. Hence parallel transport around the boundary of an annular sector is the identity. If r1,r2 are two small radii, radial parallel transport identifies the initial fibers and the endpoint fibers; the coefficient identity in [F2] makes the resulting square commute with the endpoint identifications. Thus the two Mr are conjugate and have the same characteristic polynomial.

1.2F1F3choosealgebra

Since A(reit)→R uniformly for 0≤t≤θ as r→0, subtract the integral equations for Pr(t) and eitR from [F1]. The difference norm is bounded by ∫0t∥A(reiu)−R∥ ∥Pr(u)∥ du+∫0t∥R∥ ∥Pr(u)−eiuR∥ du; Grönwall's inequality gives Pr(θ)→eiθR. Therefore Mr→geiθR.

2.1step 1.1step 1.2algebra∎

The characteristic polynomial is a polynomial in matrix entries, hence continuous. Step 1.1 makes it constant for r>0, so its common value equals the characteristic polynomial of the limit in step 1.2, namely that of geiθR.

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