Math eqs display problem

[Min-Ho Kyung]

Hello,   This is a beginning lyx user. My problem is no screen display of some frequently used math symbols such as \sum, \int, and "-". I think this is due to some screen font missing. My version of lyx is 1.1.5fix2, and I'm using it on RedHat 7.0. Please, let me know how to fix it. Thank you.     Min-Ho Kyung