Abstract
Elimination with only the necessary row exchanges will produce the triangular factorization A = LPU, with the (unique) permutation P in the middle. The entries in L are reordered in comparison with the more familiar \(A={\widehat{P}\widehat{L}\widehat{U}}\) (where \({\widehat{P}}\) is not unique). Elimination with three other starting points 1, n and n, n and n, 1 produces three more factorizations of A, including the Wiener–Hopf form UPL and Bruhat’s \(U_1\,\pi\,U_2\) with two upper triangular factors.
All these starting points are useless for doubly infinite matrices. The matrix has no first or last entry. When A is banded and invertible, we look for a new way to establish A = LPU. The key is to locate the pivot rows (we also find the main diagonal of A). LPU was previously known in the periodic (block Toeplitz) case \(A(i,j)=A(i+b,j+b)\), by factoring a matrix polynomial.
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Acknowledgements
This chapter was begun on a visit to the beautiful Hong Kong University of Science and Technology. We thank its faculty and its president Tony Chan for their warm welcome. On my final morning, George Lusztig happened to walk by—and he clarified the history of Bruhat. The books of Marko Lindner (and our email correspondence) introduced me to the limit analysis of non-Toeplitz infinite matrices.
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Strang, G. (2015). The Algebra of Elimination. In: Balan, R., Begué, M., Benedetto, J., Czaja, W., Okoudjou, K. (eds) Excursions in Harmonic Analysis, Volume 3. Applied and Numerical Harmonic Analysis. Birkhäuser, Cham. https://doi.org/10.1007/978-3-319-13230-3_1
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