## Real and complex varieties: comparisons, contrasts and examples

### Institute for Mathematics and its Applications University of Minnesota April 26, 2007

I want to begin by thanking the IMA administrators and staff for running this incredibly excellent year-long program
on Applications of Algebraic Geometry. And in the same context the organizers of the various workshops, tutorials,
and seminars also deserve to be highly commended.

Outline

Examples and properties of plane curves

 Hyperbolic paraboloid

 Hyperboloid of one sheet

 A cone asymptotic to a hyperboloid

 The hyperboloid and a tangent plane

• The quadric cone has a unique singular point, namely its vertex.

• Each of the smooth quadric ruled surfaces contains two families of straight lines.

• The intersection of the hyperboloid and the tangent plane is a reducible plane conic
-- accordingly, the union of two lines in the tangent plane.

• Every smooth quadric over C is ruled.

• Non-ruled smooth (real) quadrics: ellipsoid, elliptic paraboloid, hyperboloid of two sheets.
• Sketches not currently available
• The intersection of each surface with a tangent plane is a (singular) conic with only one real point.

Tangent surfaces of space curves.

Images under generic projection.

 Cubic ruled surface

 The Steiner surface

• Given a smooth surface in P4 (projective 4-space), generic projection to P³ refers to the process of centrally projecting it it from a generic point of P4. In the case of a smooth surface in P5, we project from a generic line in P5. {Or we can iterate the process of projecting from a point.}

• The image of a smooth surface, under generic projection to P³, is a surface with a purely 1-dimensional singular locus. Most of the singular points are ordinary double points (where two smooth sheets of the surface cross transversally). There are finitely many pinch points and finitely many triple points.

• For a complex projective variety X, a corollary of the Fulton-Hansen connectedness theorem says that pinch points always occur when X is projected to Pn, where n < 2·dim(X) - 1. {The proof of this corollary involves observing that the pre-image of the diagonal is connected if we map XX to PnPn.}

• There are very few examples of generic projections of smooth surfaces to P³ where triple points are not present. Our cubic ruled surface is one of them; it is the generic projection of a smooth surface of degree 3 in P4.

• The entire z-axis is part of the real locus of the cubic ruled surface. But only the part with -1 < z < 1 is shown. Originally, I did this because this part indicates more clearly the nature of the complex surface.

• The other portions of the z-axis, however, indicate an interesting feature of some singular real varieties:
In the neighborhood of a singular point, the local dimension of a real variety can be lower than what you might expect.

• Please  don't  do this at home.

• The Geometry Center and JGV