On the Tangent Space to the Universal Teichmuller Space
We find a remarkably simple relationship between the following two models of the tangent space to the Universal Teichm\"uller Space: (1) The real-analytic model consisting of Zygmund class vector fields on the unit circle; (2) The complex-analytic model comprising 1-parameter families of schlicht functions on the exterior of the unit disc which allow quasiconformal extension. Indeed, the Fourier coefficients of the vector field in (1) turn out to be essentially the same as (the first variations of) the corresponding power series coefficients in (2). These identities have many applications; in particular, to conformal welding, to the almost complex structure of Teichm\"uller space, to study of the Weil-Petersson metric, to variational formulas for period matrices, etc. These utilities are explored.