Abstract
In this paper, we discuss characteristic polynomials in (Clifford) geometric algebras \(\mathcal {G}_{p,q}\) of vector space of dimension \(n=p+q\). We present basis-free formulas for all characteristic polynomial coefficients in the cases \(n\le 6\), alongside with a method to obtain general form of these formulas. The formulas involve only the operations of geometric product, summation, and operations of conjugation. All the formulas are verified using computer calculations. We present an analytical proof of all formulas in the case \(n=4\), and one of the formulas in the case \(n=5\). We present some new properties of the operations of conjugation and grade projection and use them to obtain the results of this paper. We also present formulas for characteristic polynomial coefficients in some special cases. In particular, the formulas for vectors (elements of grade 1) and basis elements are presented in the case of arbitrary n, the formulas for rotors (elements of spin groups) are presented in the cases \(n\le 5\). The results of this paper can be used in different applications of geometric algebras in computer graphics, computer vision, engineering, and physics. The presented basis-free formulas for characteristic polynomial coefficients can also be used in symbolic computation.
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Notes
The Clifford conjugation is a superposition of the grade involution \(\widehat{~~}\) and the reversion \(\widetilde{~~}\). Note that some authors [19] denote the Clifford conjugation by \({\mathop {\quad }\limits ^{\overline{\quad }}}\). We do not use separate notation for the Clifford conjugation in this paper and write the combination of the two symbols \(\widehat{~~}\) and \(\widetilde{~~}\).
Note that the property (2.12) is also follows from the definition of \(\bullet \) and the fact that grade projections commute with operations of conjugation.
The commutator and anticommutator of two arbitrary elements \(U,V \in \mathcal {G}_{p,q}\) are denoted by \([U,V]=UV-VU\) and \(\{U,V\}=UV+VU\) respectively.
We remind that we use the simplified notation \(U_k:=\langle U \rangle _k\) in this paper.
Note that alternatively one could get directly \(D_{(k)}(\lambda )=\frac{1}{(N-k)!}\frac{ \partial ^{N-k} D_{(N)}(\lambda )}{ \partial \lambda ^{N-k}}\).
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Acknowledgements
The main results of this paper were reported at the International Conference “Computer Graphics International 2021 (CGI2021)” (online, Geneva, Switzerland, September 2021). The authors are grateful to the organizers and the participants of this conference for fruitful discussions. The authors are grateful to the anonymous Reviewers for their careful reading of the paper and helpful comments on how to improve the presentation. The publication was prepared within the framework of the Academic Fund Program at the HSE University in 2022 (grant 22-00-001).
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Appendix A: Basis-Free Formulas in the Case \(n=6\)
Appendix A: Basis-Free Formulas in the Case \(n=6\)
In the case \(n=6\), we have the following basis-free formulas for the characteristic polynomial coefficients \(C_{(3)}, C_{(4)}, C_{(5)}, C_{(6)}\in \mathcal {G}_{p,q}\). The remaining characteristic polynomial coefficients \(C_{(1)}, C_{(2)}, C_{(7)}, C_{(8)}\in \mathcal {G}_{p,q}\) are presented in Sect. 8.
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Abdulkhaev, K., Shirokov, D. Basis-Free Formulas for Characteristic Polynomial Coefficients in Geometric Algebras. Adv. Appl. Clifford Algebras 32, 57 (2022). https://doi.org/10.1007/s00006-022-01232-0
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DOI: https://doi.org/10.1007/s00006-022-01232-0
Keywords
- Basis-free formula
- Characteristic polynomial
- Clifford algebra
- Geometric algebra
- Grade projection
- Operation of conjugation
- Spin group