Abstract
The K-length of a form f in \(K[x_{1},\ldots,x_{n}]\), \(K \subset \mathbb{C}\), is the smallest number of d-th powers of linear forms of which f is a K-linear combination. We present many results, old and new, about K-length, mainly for n = 2, and often about the length of the same form over different fields. For example, the K-length of \(3{x}^{5} - 20{x}^{3}{y}^{2} + 10x{y}^{4}\) is three for \(K = \mathbb{Q}(\sqrt{-1})\), four for \(K = \mathbb{Q}(\sqrt{-2})\) and five for \(K = \mathbb{R}\).
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Reznick, B. (2013). On the Length of Binary Forms. In: Alladi, K., Bhargava, M., Savitt, D., Tiep, P. (eds) Quadratic and Higher Degree Forms. Developments in Mathematics, vol 31. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-7488-3_8
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