where Θ is (the sheaf of germs of sections of) the holomorphic tangent bundle. There is an obstruction in the "H"2 of the same sheaf; which is always zero in case of a curve, for general reasons of dimension. In the case of genus 0 the "H"1 vanishes, also. For genus 1 the dimension is the Hodge number "h"1,0 which is therefore 1. It is known that all curves of genus one have equations of form "y"2 = "x"3 + "ax" + "b". These obviously depend on two parameters, a and b, whereas the isomorphism classes of such curves have only one parameter. Hence there must be an equation relating those a and b which describe isomorphic elliptic curves. It turns out that curves for which "b"2"a"−3 has the same value, describe isomorphic curves. I.e. varying a and b is one way to deform the structure of the curve "y"2 = "x"3 + "ax" + "b", but not all variations of "a,b" actually change the isomorphism class of the curve.