  Propagation of fast-mode magnetohydrodynamic (MHD) waves in the interstellar
space is simulated, and focusing MHD wave (FMW) model is proposed for triggered
star formation (SF). Waves from an SF region are trapped by nearby molecular
clouds and converge onto their focal points, causing implosive compression.
Even an isolated cloud suffers from long-distance invasion of waves from remote
sources. Echoing SF occurs inside a cloud as well as between clouds. Repetitive
refocusing in a filamentary cloud suggests spatial periodicity in SF sites
along the filament. The model is applied to the SF regions M16 and M17, where
MHD waves produced by M16 are shown to converge onto the focal point of nearby
GMC and trigger the SF in M17.
