Sum-Rule Calculation of the Isovector Form Factor

A class of sum rules due to Fubini is rederived from a point of view which clarifies the assumptions made about the singularities of field operators. A calculation of the isovector magnetic moment is performed in the isobar approximation. The result $F_1^{}{}_{}{}^V\mathrm{}\left(0\right)=\frac{1}{2}$ emerges as a model-independent consistency condition, but the calculated value $F_2^{}{}_{}{}^V\mathrm{}\left(0\right)=3.64\mathrm{}$ is in disagreement with the experimental value.