This thesis explores aspects of gravity and quantum field theory (QFT). The first part is devoted to the interplay between manifest T-duality symmetry and higher-derivative corrections to the low-energy effective action of string theory. The second part focuses on exact results in supersymmetric gauge theories. We first discuss T-duality symmetry of string theory from various perspectives. We next review the manifest-duality-symmetric formulation of low-energy effective actions: double field theory (DFT) and its higher-derivative generalization, HSZ (Hohm-Siegel-Zwiebach) theory. We then compute on-shell three-point amplitudes in the HSZ theory. We show that these amplitudes factorize, as in bosonic and heterotic string theories, but they differ from both. Further, we analyze degrees of freedom in the HSZ theory. The spectrum of the theory contains massive spin-2 ghosts and massive scalars, in addition to massless fields. The massive modes can be integrated out exactly at the quadratic level, leading to an infinite series of higher-derivative corrections. Finally, we give a ghost-free massive extension of linearized DFT, which employs novel mass terms for the dilaton and the graviton. In the second part, we start by reviewing the exact results for partition functions of supersymmetric gauge theories on spheres. Exact results, however, are not available for minimally supersymmetric theories on S4 and S6. Minahan conjectured the form of perturbative partition functions for theories on Sd with eight and sixteen supersymmetries. We show that this form gives the correct one-loop divergences of the flat-space super Yang-Mills (SYM) upon taking the radius of the sphere to infinity. We also prove the conjecture explicitly for theories with eight supersymmetries. Further, we extend our results to theories with four supersymmetries for d

This monograph gives a detailed and pedagogical account of the geometry of rigid superspace and supersymmetric Yang-Mills theories. While the core of the text is concerned with the classical theory, the quantization and anomaly problem are briefly discussed following a comprehensive introduction to BRS differential algebras and their field theoretical applications. Among the treated topics are invariant forms and vector fields on superspace, the matrix-representation of the super-Poincaré group, invariant connections on reductive homogeneous spaces and the supermetric approach. Various aspects of the subject are discussed for the first time in textbook and are consistently presented in a unified geometric formalism. Requiring essentially no background on supersymmetry and only a basic knowledge of differential geometry, this text will serve as a mathematically lucid introduction to supersymmetric gauge theories.

From the reviews: "... focused mainly on complex differential geometry and holomorphic bundle theory. This is a powerful book, written by a very distinguished contributor to the field" (Contemporary Physics )"the book provides a large amount of background for current research across a spectrum of field. ... requires effort to read but it is worthwhile and rewarding" (New Zealand Math. Soc. Newsletter) " The contents are highly technical and the pace of the exposition is quite fast. Manin is an outstanding mathematician, and writer as well, perfectly at ease in the most abstract and complex situation. With such a guide the reader will be generously rewarded!" (Physicalia) This new edition includes an Appendix on developments of the last 10 years, by S. Merkulov.

From the reviews: "... focused mainly on complex differential geometry and holomorphic bundle theory. This is a powerful book, written by a very distinguished contributor to the field" (Contemporary Physics )"the book provides a large amount of background for current research across a spectrum of field. ... requires effort to read but it is worthwhile and rewarding" (New Zealand Math. Soc. Newsletter) " The contents are highly technical and the pace of the exposition is quite fast. Manin is an outstanding mathematician, and writer as well, perfectly at ease in the most abstract and complex situation. With such a guide the reader will be generously rewarded!" (Physicalia) This new edition includes an Appendix on developments of the last 10 years, by S. Merkulov.