Approximation theorems for the Schrödinger equation and quantum vortex reconnection

We prove the existence of smooth solutions to the Gross¿Pitaevskii equation on R3 that feature arbitrarily complex quantum vortex reconnections. We can track the evolution of the vortices during the whole process. This permits to describe the reconnection events in detail and verify that this scenario exhibits the properties observed in experiments and numerics, such as the t1/2 and change of parity laws. We are mostly interested in solutions tending to 1 at infinity, which have finite Ginzburg¿Landau energy and physically correspond to the presence of a background chemical potential, but we also consider the cases of Schwartz initial data and of the Gross¿Pitaevskii equation on the torus. In the proof, the Gross¿Pitaevskii equation operates in a nearly linear regime, so the result applies to a wide range of nonlinear Schrödinger equations. Indeed, an essential ingredient in the proofs is the development of novel global approximation theorems for the Schrödinger equation on Rn. Specifically, we prove a qualitative approximation result that applies for solutions defined on very general spacetime sets and also a quantitative result for solutions on product sets in spacetime D × R. This hinges on frequencydependent estimates for the Helmholtz¿Yukawa equation that are of independent interest.Acuerdo CRUE-CSIC con Springer Nature; A.E. is supported by the ERC Consolidator Grant 862342. D.P.-S. is supported by the grants MTM PID2019-106715GB-C21 (MICINN) and Europa Excelencia EUR2019-103821 (MCIU). This work is supported in part by the ICMAT–Severo Ochoa grant CEX2019-000904-S