Computation of real-valued basis functions which transform as irreducible representations of the polyhedral groups

TL;DR

This paper proves the existence of real-valued basis functions transforming as irreducible representations of polyhedral groups, providing algorithms for icosahedral and octahedral symmetries, with implications for cryo-electron microscopy.

Contribution

It demonstrates the existence of real-valued basis functions for certain polyhedral groups and offers algorithms for their computation, advancing symmetry-based analysis.

Findings

Real-valued basis functions exist for icosahedral and octahedral groups.

Such bases do not exist for tetrahedral symmetry due to complex irreps.

Algorithms for computing these bases are provided for specific symmetries.

Abstract

Basis functions which are invariant under the operations of a rotational point group $G$ are able to describe any 3-D object which exhibits the rotational point group symmetry. However, in order to characterize the spatial statistics of an ensemble of objects in which each object is different but the statistics exhibit the symmetry, a complete set of basis functions is required. In particular, for each irreducible representation (irrep) of $G$, it is necessary to include basis functions that transform according to that irrep. This complete set of basis functions is a basis for square-integrable functions on the surface of the sphere in 3-D. Because the objects are real-valued, it is convenient to have real-valued basis functions. In this paper, the existence of such real-valued bases is proven and an algorithm for their computation is provided for the icosahedral $I$ and the octahedral $O$ symmetries. Furthermore, it is proven that such a real-valued basis does not exist for the tetrahedral $T$ symmetry because some irreps of $T$ are essentially complex. The importance of these basis functions to computations in single-particle cryo-electron microscopy is described.