Hi, Christophe and Jean-Francois It is the version of cygwin that cause the trouble. After I updated to 1.3.2, it works fine. Thank you for your help. I am still puzzled by those seperated wavefronts. Can you suggest any idea why it looks like so? Lin -- Dept. of Math., RPI 518-276,2184(work) 518-271,4486(home)