  The n-instanton contribution to the Seiberg-Witten prepotential of N=2
supersymmetric d=4 Yang Mills theory is represented as the integral of the
exponential of an equivariantly exact form. Integrating out an overall scale
and a U(1) angle the integral is rewritten as (4n-3) fold product of a closed
two form. This two form is, formally, a representative of the Euler class of
the Instanton moduli space viewed as a principal U(1) bundle, because its
pullback under bundel projection is the exterior derivative of an angular
one-form.
