Derivation of the exact NSVZ beta-function in N=1 SQED, regularized by   higher derivatives, by direct summation of Feynman diagrams
  For N=1 supersymmetric quantum electrodynamics, regularized by higher derivatives, a method for summation of all Feynman diagrams defining the beta-function is presented. Using this method we prove that the beta-function is given by an integral of a total derivative, which can be easily calculated. It is shown that surviving terms give the exact NSVZ beta-function. The results are compared with the explicit three-loop calculation. 
K.V. Stepanyantz
hep-th