The Properties of Your Cubic Surface

The Properties of Your Cubic Surface

describing the properties of your cubic surface and related curves including appropriate computer-generated images. Please make your title interesting and include an abstract at the beginning of the paper. Submit as pdfs and electronic files in CANVAS (i.e. pdf+ source MSword, Latex files, (as text) , ppt, etc. Include your computer codes if you use them for calculations.

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  1. Title, name, abstract – (write this part at the end highlighting your results).
  2. Give basic definitions of P3 (x,y,z,w), an algebraic variety in P3, an irreducible variety, singular points, a dimension a variety.
  3. Give the homogenous equation f of your surface V in P3 (x,y,z,w). What is the space classifying all degree 3 surfaces in P3? What is the dimension of this classifying space? Show the calculations.
  4. Give definitions of Ux, Uy, Uz, Uw. . Graph your surface for w=1 , x=1, y=1, z=1 (use Surfer or any other graphing software).


  1. Find singular points of V in P3 or prove there are none. Find the inflection points on your surface or prove there are none.


  1. Pick a non-singular point and write the equation of a tangent space. What dimension is your variety at non-singular points?
  2. Calculate Gaussian curvature of your surface V given by the equation F(x,y,z) = 0 in R3 = Uw,  at least two non-singular points. Analyze the changes of the curvature on your surface? Is it always positive or negative? Is there a curve when the curvature equals always to 0?
  3. Describe the ideal I(V). Is it prime? Is your variety irreducible in P3 (justify)?
  4. Describe the ring O(V) of regular functions on your surface. Describe the field of rational functions K(V). Is your V birational to P2?
  5. Describe symmetries of your V, Aut(V) – give generators or  matrices if possible.

You can consult these:

Example: Aut (Sphere} = (mP , rl,a id, where P is any plane passing through, the center, l is any line passing through the center, and a is any angle }  is infinite, and can be described by the group of orthogonal 3 x 3  matrices  O(3).



  1. Find lines on your variety by solving the equations in variables s and t (you may use computers) or justify that there are none.
  2. Consider curves (divisors) on V given by V intersected with a plane x=0, y=0, z=0, w= 0. Calculate genus of each curve, if possible.

(The genus formula for a smooth curve on a plane is  g= (d-1)(d-2)/2 ,     where d is a degree of the polynomial defining the curve).

  1. What can you say about the family of curves given by equations x=a (degree, irreducibility, singularities, genus,  etc.)? What can you say about the family of curves given by equations y=b?    What can you say about the family of curves given by equations z=c? What can you say about the family of curves given by equations w=d?
  2. Are there any other interesting curves that lie on your variety V (i.e. not plane sections)? For example, a twisted cubic or an elliptic curve (g=1) may lie on your surface.
  3. Define a family of (interesting) deformations of V parameterized  by a in R1.  What happens to irreducibility, singularities, symmetries, lines, etc.? What happens when the parameter agoes to +infinity, – infinity? Show appropriate images.

Bibliography – cite all sources you have used, including our textbook and a calc book


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