Cubic form

In mathematics, a cubic form is a homogeneous polynomial of degree 3, and a cubic hypersurface is the zero set of a cubic form. In the case of a cubic form in three variables, the zero set is a cubic plane curve.

In (Delone & Faddeev 1964), Boris Delone and Dmitry Faddeev showed that binary cubic forms with integer coefficients can be used to parametrize orders in cubic fields. Their work was generalized in (Gan, Gross & Savin 2002, §4) to include all cubic rings,^[1]^[2] giving a discriminant-preserving bijection between orbits of a GL(2, Z)-action on the space of integral binary cubic forms and cubic rings up to isomorphism.

The classification of real cubic forms $ax^{3}+3bx^{2}y+3cxy^{2}+dy^{3}$ is linked to the classification of umbilical points of surfaces. The equivalence classes of such cubics form a three-dimensional real projective space and the subset of parabolic forms define a surface – the umbilic torus or umbilic bracelet.^[3]

Examples

^ A cubic ring is a ring that is isomorphic to Z³ as a Z-module.
^ In fact, Pierre Deligne pointed out that the correspondence works over an arbitrary scheme.
^ Porteous, Ian R. (2001), Geometric Differentiation, For the Intelligence of Curves and Surfaces (2nd ed.), Cambridge University Press, p. 350, ISBN 978-0-521-00264-6

Delone, Boris; Faddeev, Dmitriĭ (1964) [1940, Translated from the Russian by Emma Lehmer and Sue Ann Walker], The theory of irrationalities of the third degree, Translations of Mathematical Monographs, vol. 10, American Mathematical Society, MR 0160744
Gan, Wee-Teck; Gross, Benedict; Savin, Gordan (2002), "Fourier coefficients of modular forms on G₂", Duke Mathematical Journal, 115 (1): 105–169, doi:10.1215/S0012-7094-02-11514-2, MR 1932327
Iskovskikh, V.A.; Popov, V.L. (2001) [1994], "Cubic form", Encyclopedia of Mathematics, EMS Press
Iskovskikh, V.A.; Popov, V.L. (2001) [1994], "Cubic hypersurface", Encyclopedia of Mathematics, EMS Press
Manin, Yuri Ivanovich (1986) [1972], Cubic forms, North-Holland Mathematical Library, vol. 4 (2nd ed.), Amsterdam: North-Holland, ISBN 978-0-444-87823-6, MR 0833513

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