2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/51745The non-perturbative (D-instanton corrected) low-energy effective action of the universal hypermultiplet in the type-IIA string theory compactified on a Calabi-Yau threefold is calculated in 4d, N=2 supergravity. The action is given by the quaternionic non-linear sigma-model whose metric is governed by the Eisenstein series E_{3/2}. The U(1)xU(1) isometry and SL(2,Z) modular invariance play the key role in our construction.7 pages, invited talk at the YITP String Theory Workshop in Kyoto, November 15-16, 2002High Energy Physics - Theorytext