Projective varieties

Today I will try and study projective varieties and their ideals. Although my understanding of these objects has improved over time, there are still a lot of chinks that need to be filled.

Something that has defied complete understanding is what kinds of polynomials are projective varieties the zeroes of? Do these polynomials have to be homogeneous? The point is that each homogeneous component has to satisfy the zeroes independently. Hence, a polynomial corresponding to a projective variety need not be homogeneous. In fact, a sum of homogeneous polynomials, possibly of different degrees, satisfies a smaller algebraic set than each of the individual components do. This follows from what is written above.

What does the ideal corresponding to an affine projective variety look like? Why does it have to be generated by homogeneous polynomials? It does not have to be generated by homogeneous polynomials. However, it *can* be. This is because each of the generators, of which there are finite, can be broken down into their homogeneous components. And it is easy to see that the ideal generated by the homogeneous components is the same as that generated by the generators mentioned above.

What does the coordinate ring of a projective variety look like? It is constructed in exactly the same way that the coordinate ring of an affine variety is constructed. The only difference is that the ideal, which we quotient the polynomial ring k[x_1,x_2,\dots,x_n] by, is homogeneous in this case.

But what about the non-homogeneous polynomials in this coordinate ring? Are they even well defined over a projective variety? This is a question I will get back to and resolve as soon as I can.

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Graduate student

One thought on “Projective varieties

  1. It’s not clear to me what you are talking about in the second paragraph. Some explicit examples might help. What exactly do you mean by a polynomial corresponding to a projective variety? If a polynomial f is not homogenous, then how do you make sure f(x) = 0 implies f(\lambda x) = 0 for all \lambda? In particular, talking about zeros of a non-homogeneous polynomial does not make much sense in projective spaces, unless you talk about the projective closure.

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