2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/153881In this letter, I have considered one-dimensional quantum system with different masses $m$ and $M$, which does not appear integrable in general. However I have found an exact two-body wave function and due to the extension of the integrable system to more general system, it was concluded that the rapidity or quasi-momentum in the integrable system should be regarded as a modification of velocity rather than that of momentum. I have also considered the three-body wave function and argued its integrable condition.AMS-Tex UseExactly Solvable and Integrable Systemstext