MHB Quadratic Forms: Beyond Sketching Conics

matqkks
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What are the real life applications of quadratic forms? I have used them to sketch conics but are there any other applications?
 
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One of the more important (fundamental) optimization problems in control theory is the linear-quadratic regulator problem, in which you try to solve a linear DE subject to a quadratic constraint. The quadratic constraint involves quadratic forms.
 
I have used quadratic forms to match the digital image of a pupil (in perspective) with an ellipse.

A friend of mine worked on an application of quadric forms (3 dimensional) to match a heart in an MRI scan (3D image) with an ellipsoid.
 
The world of 2\times 2 complex matrices is very colorful. They form a Banach-algebra, they act on spinors, they contain the quaternions, SU(2), su(2), SL(2,\mathbb C), sl(2,\mathbb C). Furthermore, with the determinant as Euclidean or pseudo-Euclidean norm, isu(2) is a 3-dimensional Euclidean space, \mathbb RI\oplus isu(2) is a Minkowski space with signature (1,3), i\mathbb RI\oplus su(2) is a Minkowski space with signature (3,1), SU(2) is the double cover of SO(3), sl(2,\mathbb C) is the...
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