# Plane conic constructor¶

AUTHORS:

• Marco Streng (2010-07-20)
• Nick Alexander (2008-01-08)
sage.schemes.plane_conics.constructor.Conic(base_field, F=None, names=None, unique=True)

Return the plane projective conic curve defined by F over base_field.

The input form Conic(F, names=None) is also accepted, in which case the fraction field of the base ring of F is used as base field.

INPUT:

• base_field – The base field of the conic.

• names – a list, tuple, or comma separated string of three variable names specifying the names of the coordinate functions of the ambient space $$\Bold{P}^3$$. If not specified or read off from F, then this defaults to 'x,y,z'.

• F – a polynomial, list, matrix, ternary quadratic form, or list or tuple of 5 points in the plane.

If F is a polynomial or quadratic form, then the output is the curve in the projective plane defined by F = 0.

If F is a polynomial, then it must be a polynomial of degree at most 2 in 2 variables, or a homogeneous polynomial in of degree 2 in 3 variables.

If F is a matrix, then the output is the zero locus of $$(x,y,z) F (x,y,z)^t$$.

If F is a list of coefficients, then it has length 3 or 6 and gives the coefficients of the monomials $$x^2, y^2, z^2$$ or all 6 monomials $$x^2, xy, xz, y^2, yz, z^2$$ in lexicographic order.

If F is a list of 5 points in the plane, then the output is a conic through those points.

• unique – Used only if F is a list of points in the plane. If the conic through the points is not unique, then raise ValueError if and only if unique is True

OUTPUT:

A plane projective conic curve defined by F over a field.

EXAMPLES:

Conic curves given by polynomials

sage: X,Y,Z = QQ['X,Y,Z'].gens()
sage: Conic(X^2 - X*Y + Y^2 - Z^2)
Projective Conic Curve over Rational Field defined by X^2 - X*Y + Y^2 - Z^2
sage: x,y = GF(7)['x,y'].gens()
sage: Conic(x^2 - x + 2*y^2 - 3, 'U,V,W')
Projective Conic Curve over Finite Field of size 7 defined by U^2 + 2*V^2 - U*W - 3*W^2


Conic curves given by matrices

sage: Conic(matrix(QQ, [[1, 2, 0], [4, 0, 0], [7, 0, 9]]), 'x,y,z')
Projective Conic Curve over Rational Field defined by x^2 + 6*x*y + 7*x*z + 9*z^2

sage: x,y,z = GF(11)['x,y,z'].gens()
sage: C = Conic(x^2+y^2-2*z^2); C
Projective Conic Curve over Finite Field of size 11 defined by x^2 + y^2 - 2*z^2
sage: Conic(C.symmetric_matrix(), 'x,y,z')
Projective Conic Curve over Finite Field of size 11 defined by x^2 + y^2 - 2*z^2


Conics given by coefficients

sage: Conic(QQ, [1,2,3])
Projective Conic Curve over Rational Field defined by x^2 + 2*y^2 + 3*z^2
sage: Conic(GF(7), [1,2,3,4,5,6], 'X')
Projective Conic Curve over Finite Field of size 7 defined by X0^2 + 2*X0*X1 - 3*X1^2 + 3*X0*X2 - 2*X1*X2 - X2^2


The conic through a set of points

sage: C = Conic(QQ, [[10,2],[3,4],[-7,6],[7,8],[9,10]]); C
Projective Conic Curve over Rational Field defined by x^2 + 13/4*x*y - 17/4*y^2 - 35/2*x*z + 91/4*y*z - 37/2*z^2
sage: C.rational_point()
(10 : 2 : 1)
sage: C.point([3,4])
(3 : 4 : 1)

sage: a=AffineSpace(GF(13),2)
sage: Conic([a([x,x^2]) for x in range(5)])
Projective Conic Curve over Finite Field of size 13 defined by x^2 - y*z


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Projective plane conics over a field