Ternary quartic

In mathematics, a ternary quartic form is a degree 4 homogeneous polynomial in three variables.

Hilbert's theorem

showed that a positive semi-definite ternary quartic form over the reals can be written as a sum of three squares of quadratic forms.

Invariant theory

The ring of invariants is generated by 7 algebraically independent invariants of degrees 3, 6, 9, 12, 15, 18, 27, together with 6 more invariants of degrees 9, 12, 15, 18, 21, 21, as conjectured by. discussed the invariants of order up to about 15.
The Salmon invariant is a degree 60 invariant vanishing on ternary quartics with an
inflection bitangent.

Catalecticant

The catalecticant of a ternary quartic is the resultant of its 6 second partial derivatives. It vanishes when the ternary quartic can be written as a sum of five 4th powers of linear forms.