PUB - Publikationen an der Universität Bielefeld

The calculation of Feynman diagrams in terms of a Taylor expansion w.r.t. small momenta squared is a very promising method and may become particularly important for the higher loop diagrams. If the lowest thresholds are very low, however, then the Taylor series expansion is either difficult to apply or extremely many Taylor coefficients might be needed to achieve convergence. In the case of one variable, q(2), the relevant normalization is r = q(2)/q(th)(2) (q(th)(2) = threshold value of q(2)) and we demonstrate that with 30 Taylor coefficients up to r = 100 a precision of 3 to 4 decimals can be achieved. For the case of a low threshold we compare our results with the zero threshold case obtained by first applying a large mass expansion. bs expected the deviation is small which serves as an excellent test for both the methods.