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Algebraic and geometric theory of quadratic forms and symmetric bilinear forms, e.g., values attained by quadratic forms, isotropic subspaces, the Witt ring, invariants of quadratic forms, the discriminant and Clifford algebra of a quadratic form, Pfister forms, automorphisms of quadratic forms.

4 votes

how to determine a biquadratic form is positive-definite

In general one doesn't expect to have nice necessary and sufficient conditions for checking positivity of a biquadratic form. The sum-of-squares methods outlined in these course notes provide an effi …
Noah Stein's user avatar
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10 votes
Accepted

Characterising semi-definite positiveness on vectors with non-negative entries

The cone $C$ is called the cone of copositive matrices and its dual $C^*$ is called the cone of completely positive matrices. Here are some references. The paper most relevant to your question is pr …
Noah Stein's user avatar
  • 8,501
4 votes
Accepted

Decide how many non-negative solutions a set of multivariate quadratic equations have

Not efficiently, at least not unless the problem has some additional structure which can be exploited. The set of mixed Nash equilibria of a two-player game can be written as the nonnegative solution …
Noah Stein's user avatar
  • 8,501
2 votes

Solving a System of Quadratic Equations

What you have is an instance of a quadratically constrained quadratic program (QCQP). These problems are NP-hard in general (though it's possible your particular type of instance is not hard as fedja …
Noah Stein's user avatar
  • 8,501