This v2 is only up to date as far as November 2024. Excepting that its references were updated in December 2025.
Newsflash: this article is being split into the body, keeping its original name, for Annals of the Instiute of the Theory STEM. And its Appendix, to feature as a Review “Eigentheory in Detail: Real-Symmetric Subcase” for the Online Encyclopedia of Tensors and other Multi-linear Arrays.
Concepts of Shape 