1. Real Ghosts of Complex Hadamard Products.
- Author
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Ballico, Edoardo
- Subjects
- *
COMPLEX numbers , *SYMMETRY groups , *OPEN-ended questions , *INTEGERS , *STATISTICS - Abstract
For all integers n ≥ 1 and k ≥ 2 , the Hadamard product v 1 ★ ⋯ ★ v k of k elements of K n + 1 (with K being the complex numbers or real numbers) is the element v ∈ K n + 1 which is the coordinate-wise product of v 1 , ... , v k (introduced by Cueto, Morton, and Sturmfels for a model in Algebraic Statistics). This product induces a rational map h : P n (K) k ⤏ P n (K) . When K = C , k = 2 and X i (C) ⊂ P n (C) , i = 1 , 2 are irreducible, we prove four theorems for the case dim X 2 (C) = 1 , three of them with X 2 (C) as a line. We discuss the existence (non-existence) of a cancellation law for ★-products and use the symmetry group of the Hadamard product. In the second part, we work over R. Under mild assumptions, we prove that by knowing X 1 (R) ★ ⋯ ★ X k (R) , we know X 1 (C) ★ ⋯ ★ X k (C) . The opposite, i.e., taking and multiplying a set of complex entries that are invariant for the complex conjugation and then seeing what appears in the screen P n (R) , very often provides real ghosts, i.e., images that do not come from a point of X 1 (R) × ⋯ × X k (R) . We discuss a case in which we certify the existence of real ghosts as well as a few cases in which we certify the non-existence of these ghosts, and ask several open questions. We also provide a scenario in which ghosts are not a problem, where the Hadamard data are used to test whether the images cover the full screen. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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