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Certified Golub–Welsch intervals contain the Gaussian nodes and weights

Statement

Let admissible moment data (a,b,m,r,μ) with r≥2m−1 and finite moment intervals satisfying the promise in Certified Gaussian nodes and weights by Golub–Welsch be given. If that algorithm returns certified with node intervals Ni and weight intervals Wi, then the Gaussian nodes xi and weights wi satisfy xi∈Ni and wi∈Wi for every i=1,…,m. An inconclusive return makes no claim.

Facts & Assumptions

Given: Admissible moment data, truthful moment enclosures through degree 2m−1, and a certified algorithm return.

[F1]

The uniform interval residual certifier is sound on success: on certified output, each supplied Hankel family has a unique solution and its coefficients lie in the reported error balls.

[F2]

Outward-rounded operations are inclusion-isotone and enclose DAG evaluations: all finite outward interval expression DAGs contain the corresponding exact values.

[F3]

The Gauss orthogonal polynomial has simple roots inside the interval and The Jacobi matrix spectrum is the set of Gauss nodes: the true Jacobi matrix is real symmetric and its increasingly ordered eigenvalues are exactly the Gaussian nodes in (a,b).

[F4]

Certified Hermitian eigenvalue intervals from residual and Gram bounds and The residual and orthogonality certifier encloses the full ordered spectrum: success requires validated residual, Gram and norm bounds, and every increasingly ordered eigenvalue in the admitted Hermitian matrix family lies in its returned node interval.

[F5]

Gaussian weights equal the total mass times a Jacobi eigenvector's first-coordinate square: for a unit eigenvector ui at xi, wi=μ0∣(ui)1∣2.

[F6]

A residual and spectral gap bound a Gaussian weight error: a nonzero approximate eigenvector with norm lower bound s, residual bound ρ, and separation gap lower bound gi gives weight error at most 22 μ0ρ/(sgi).

Proof

1.1F1F2givenalgebra

Each coefficient solve in step 1 uses the full moment rectangle families, which contain the actual Hankel matrix and right-hand side by the moment promise. By [F1], successful uniform residual certification encloses the unique actual coefficient vector. The true Hankel matrices are positive definite by the support hypothesis and The polynomial moment pairing is a real inner product; thus these solutions are the coefficients of the monic orthogonal polynomials. By [F2], the arithmetic in step 2 encloses every true hj,αj,βj and hence the true Jacobi matrix.

2.1F2F3F5step 1.1algebra

If m=1, the true Jacobi matrix is the one-by-one matrix [α0]. By [F3] its sole eigenvalue is the sole Gaussian node x1, so x1=α0. Its unit eigenvector is (1), and [F5] gives w1=μ0. The outward interval α0 and moment interval I0 in step 3 of the algorithm contain these exact values by [F2].

2.2F3F4step 1.1

Assume m≥2. By [F3], the eigenvalues of the true Jacobi matrix are precisely the Gaussian nodes. The matrix family in step 5 contains this matrix by step 1.1, and successful near-orthonormal certification gives xi∈Ni by [F4].

3.1F2F4step 2.2algebra

For column vi of V, the accepted checks in step 5 and [F4] certify ∥Jvi−θivi∥2≤ρ and ∥V∗V−I∥2≤d<1. Therefore ∥vi∥22≥1−d, so the positive lower endpoint s of the outward enclosure of 1−d is a lower bound on its norm. The disjoint ordered intervals in step 5 make gi in step 6 a positive lower bound for the distance from θi to every other true node.

4.1F2F5F6step 3.1algebra

Apply [F6] to (vi,θi) and the bounds of step 3.1. The interval Ci contains μ0(vi)12/∥vi∥22 and Ri is at least the resulting error bound, because sup⁡(I0)≥μ0. Thus the exact Gaussian weight wi belongs to Ci+[−Ri,Ri]=Wi by [F5].

5.1givenstep 2.1step 2.2step 4.1cases∎

The m=1 case is step 2.1; for m≥2, steps 2.2 and 4.1 give every node and weight enclosure. These cases exhaust m≥1, proving the claim.

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Cited to discharge well-definedness by Certified Gaussian nodes and weights by Golub–Welsch.

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