2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/75093We study how Gromov-Witten invariants of projective 3-folds transform under a standard flop and a small extremal transition in the algebro-geometric setting from the recent development of algebraic relative Gromov-Witten theory and its applications. This gives an algebro-geometric account of Witten's wall-crossing formula for correlation functions of the descendant nonlinear sigma model in adjacent geometric phases of a gauge linear sigma model and of the symplectic approach in an earlier work of An-Min Li and Yongbin Ruan on the same problem. A terse account from the stringy and the symplectic viewpoint is given in the appendix to complement and compare to the discussion in the main text.38+2 pages, 5 figuresAlgebraic GeometryHigh Energy Physics - TheorySymplectic Geometry14N35; 53D45, 81T30text