2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/53931We study the free energy of the pure glue QCD string with a torus target space and the gauge groups $SU(N)$ and (chiral) $U(N)$. It is highly constrained by a strong/weak gauge coupling duality which results in modular covariance. The string free energy is computed exactly in terms of modular forms for worldsheet genera 1 - 8. It has a surprisingly mild singularity in the weak gauge coupling/small area limit.22 pages, RU-94-58High Energy Physics - Theorytext