SIGMA 22 (2026), 077, 23 pages      arXiv:2510.21186      https://doi.org/10.3842/SIGMA.2026.077

Weingarten Calculus with Virtual Isometries

Benoît Collins ^a and Sho Matsumoto ^b
a) Department of Mathematics, Kyoto University, Japan
^b) Graduate School of Science and Engineering, Kagoshima University, Japan

Received February 24, 2026, in final form August 10, 2026; Published online August 18, 2026

Abstract
In this paper, we develop a novel approach to the Weingarten calculus by employing the notion of virtual isometries. Traditionally, Weingarten calculus provides explicit formulas for integrating polynomial functions over compact matrix groups with respect to the Haar measure, yet it faces limitations when evaluating high-degree integrals due to the non-invertibility of the associated matrices. We revisit these classical computations from a new perspective: by constructing Haar-distributed matrices as products of sequences of complex reflections, we derive new recursive structures for the Weingarten functions across different dimensions. This framework leads to two main results: (1) an explicit Weingarten calculus for complex reflections, yielding systematic moment computations for associated rank-one matrices, and (2) a novel convolution formula that connects Weingarten functions in dimension $n$ to those in dimension $n-1$, through the introduction of ascension functions in the symmetric group algebra. Our approach not only provides a unified treatment for unitary groups, but also sheds light on the algebraic and probabilistic aspects of high-degree integral computations. We present several examples and applications.

Key words: unitary group; Haar measure; Weingarten calculus; virtual isometry; complex reflection.

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