Upper block triangular form for the Laplace-Beltrami operator on the special orthogonal group acquired through a flag of trace polynomials spaces

Abstract: The Laplacian of a general trace polynomial function defined on the special orthogonal group $SO(N)$ is explicitly computed. An invariant flag of spaces generated by trace polynomials is constructed. The matrix of the Laplace-Beltrami operator on $SO(N)$ for this flag of vector spaces takes an upper block triangular form. As a consequence of this construction, the eigenvalues and eigenfunctions of the Laplace-Beltrami operator on $SO(N)$ can be computed in an iterative manner. For the particular cases of the special orthogonal groups $SO(3)$ and $SO(4)$ the complete list of eigenvalues is obtained and the corresponding irreducible characters of the representation for these groups are expressed as trace polynomials.

• The paper presents a computational framework for analyzing the Laplace-Beltrami operator on SO(N) through a flag of invariant trace polynomial spaces.
• It demonstrates that the operator’s matrix representation is upper block triangular, enabling iterative computation of eigenvalues and the derivation of irreducible characters.
• Detailed cases for SO(3) and SO(4) highlight the interplay between harmonic analysis, matrix calculus, and representation theory.

Upper Block Triangular Form for the Laplace-Beltrami Operator on $SO(N)$ via Trace Polynomial Flags

Overview and Motivation

This paper presents a computational framework for analyzing the Laplace-Beltrami operator $\Delta_{SO(N)}$ on the special orthogonal group $SO(N)$, focusing on its action on spaces generated by trace polynomials. The authors construct an explicit flag of invariant subspaces, each spanned by products of power sum polynomials (trace polynomials), and demonstrate that the matrix representation of $\Delta_{SO(N)}$ with respect to this flag is upper block triangular. This structure enables iterative computation of eigenvalues and eigenfunctions, and provides explicit trace polynomial expressions for irreducible characters of $SO(N)$, with detailed results for $SO(3)$ and $SO(4)$.

Laplace-Beltrami Operator on $SO(N)$ and Spherical Eigenvalues

The Laplace-Beltrami operator on $SO(N)$ is expressed in ambient Euclidean coordinates, leveraging previous results for constrained manifolds. The explicit formula involves the Euclidean Laplacian, gradient, and Hessian, together with a matrix $\Lambda(U)$ encoding the group structure. The authors show that, via Riemannian submersion from $SO(N)$ to the sphere $S^{N-1}$, the eigenvalues $-\frac{k(k+N-2)}{2}$ for $k\in\mathbb{N}$, which are classical for the sphere, are also present in the spectrum of $\Delta_{SO(N)}$. Moreover, the corresponding eigenfunctions are Gegenbauer polynomials in the matrix entries $u_{ij}$, establishing a direct link between harmonic analysis on spheres and the spectral theory of $SO(N)$.

Construction of the Trace Polynomial Flag

A central contribution is the construction of a flag of subspaces

$$V_0 \subseteq V_{\leq 1} \subseteq \cdots \subseteq V_{\leq k} \subseteq \cdots$$

where $V_{\leq k}$ is spanned by products of trace polynomials $$p_{\boldsymbol{\lambda}}(U) = \prod_{i=1}^s \operatorname{tr}(U^{m_i})$$ for integer partitions $\boldsymbol{\lambda} \vdash j$ with $j \leq k$. The authors prove that $\Delta_{SO(N)}$ preserves each $V_{\leq k}$, and that its matrix representation in a suitable basis is upper block triangular. This result is established via explicit formulas for the Laplacian of products of trace polynomials, using matrix calculus and properties of the group.

The upper block triangular structure implies that the spectrum of $\Delta_{SO(N)}$ restricted to $\mathcal{V} = \bigcup_{k=0}^\infty V_{\leq k}$ can be computed iteratively, with the eigenvalues of each diagonal block corresponding to those of the operator on $V_{\leq k}$.

Explicit Computations for $SO(3)$

For $SO(3)$, the authors provide two natural bases for $V_{\leq k}$: one in terms of powers of $\operatorname{tr}(U)$, and another in terms of $\operatorname{tr}(U^m)$. They show that all trace polynomials can be expressed as polynomials in $\operatorname{tr}(U)$, due to the structure of $SO(3)$ eigenvalues. The matrix of $\Delta_{SO(3)}$ in these bases is upper triangular, and the eigenvalues are explicitly computed as $-\frac{k(k+1)}{2}$ for $k\in\mathbb{N}$, matching the representation-theoretic spectrum.

Irreducible characters of $SO(3)$ are shown to be eigenfunctions of $\Delta_{SO(3)}$, and are given by explicit trace polynomial formulas:

$$\chi_k(U) = \sum_{j=1}^k \operatorname{tr}(U^j) - k + 1 = \sum_{j=0}^k \left(\sum_{l=j}^k (-1)^{k-l} \binom{k+l}{2l} \binom{l}{j}\right) (\operatorname{tr}(U))^j$$

This provides a direct computational link between spectral theory and representation theory.

Spectrum and Characters for $SO(4)$

The case of $SO(4)$ is more intricate due to its non-simple structure. The authors use Riemannian submersion techniques and the relationship between $SO(4)$ and $SU(2)\times SU(2)$ to derive the spectrum:

$$\operatorname{spec} \Delta_{SO(4)} = \left\{ -\frac{1}{4} \left( k_1(k_1+2) + k_2(k_2+2) \right) \mid k_1, k_2 \in \mathbb{N},\ k_1, k_2\ \text{same parity} \right\}$$

They construct bases for $V_{\leq k}$ in terms of products of $\operatorname{tr}(U)$ and $\operatorname{tr}(U^2)$, and provide explicit matrix representations of $\Delta_{SO(4)}$ up to $k=4$. The corresponding eigenvectors are irreducible characters, which are written as trace polynomials. The approach generalizes to higher $k$, though computations become increasingly complex.

Technical Results and Computational Tools

The paper includes detailed technical lemmas on matrix calculus, including explicit formulas for gradients and Hessians of trace polynomials, commutation relations, and product rules for the Laplacian on Riemannian manifolds. These results are essential for the explicit computations and for establishing the invariance of the trace polynomial flag.

Implications and Future Directions

The explicit upper block triangular structure of $\Delta_{SO(N)}$ on trace polynomial flags provides a powerful computational tool for spectral analysis on $SO(N)$. It enables iterative and symbolic computation of eigenvalues and eigenfunctions, and offers direct formulas for irreducible characters in terms of trace polynomials. This approach bridges harmonic analysis, matrix calculus, and representation theory, and is particularly valuable for applications in mathematical physics, signal processing on manifolds, and numerical analysis of PDEs on Lie groups.

Potential future developments include extension to other compact Lie groups, analysis of multiplicities and explicit eigenbasis construction, and applications to non-commutative harmonic analysis and quantum information theory. The computational framework may also facilitate efficient algorithms for spectral decomposition and for solving differential equations on matrix groups.

Conclusion

This work provides a comprehensive computational framework for the spectral analysis of the Laplace-Beltrami operator on $SO(N)$, leveraging the structure of trace polynomial flags and their invariance. The upper block triangular form of the operator enables explicit and iterative computation of eigenvalues and eigenfunctions, with direct applications to representation theory and harmonic analysis. The results for $SO(3)$ and $SO(4)$ illustrate the effectiveness of the approach, and the technical tools developed are broadly applicable in the study of differential operators on Lie groups.