New calculation of the three-photon-annihilation contribution to the positronium hyperfine interval

Abstract
The three-photon-annihilation contribution to the positronium hyperfine interval is obtained in analytic form. This contribution is of order mα6, and has the value -0.969 MHz. The result of a previous calculation of this contribution is shown to be incomplete because of a subtlety in the regularization of infrared divergences.