Wednesday, 23 May 2007

complex geometry - Tensor product of a line bundle with a large multiple of another positive line bundle also positive?

Let us prove that for an affine variety XX every line bundle EE is "positive" according to the chosen defintion. All we need to prove is that for any hermitian metric gg on EE with curvature ww there is a Kahler form w1w1 on XX such that w1>ww1>w. Since XX is affine, for any w1w1 we have w1=fraci2pipartialbarpartial(f1)w1=fraci2pipartialbarpartial(f1) and changing the metric gg on EE by gef1gef1 we corresponing curvature will change from ww to w+w1w+w1, which we assume to be positive.



So we need to show the existence of arbitrary large w1w1. Since XX is affine and hence admits an embedding in mathbbCnmathbbCn, it is enough to show this for mathbbCnmathbbCn. Moreover, since mathbbCn=mathbbC1times...timesmathbbC1mathbbCn=mathbbC1times...timesmathbbC1 it is enought to prove the statement for mathbbC1mathbbC1. Now, on mathbbC1mathbbC1 every form of the shape w1=h1dzwedgedbarzw1=h1dzwedgedbarz is Kahler for h1>0h1>0 and we can chose h1h1 as large as we wish.



The conclusion is that if one choses this definition, then each line bundle on an affine XX is positive, which sounds strange. So I am not sure what should be a reasonable definition of positivincess in non-compact case, if it exists at all.

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