On a theorem of Rickards

A recent result of Rickards states that the generating series of intersection numbers of real quadratic geodesics on indefinite Shimura curves are elliptic modular forms. We reinterpret this as a Kudla-Millson theta series, and prove that Rickards’ generating series is the diagonal restriction of a Hilbert modular form, analogous to results of Darmon-Pozzi-Vonk and Branchereau.