Take a math matrix, 102 U.S. cities and a few air travel stats, stir in some other arcana and...presto! A prediction (80 percent accurate) of rates and locations of AIDS cases in the year 2003. The formula-an equation whipped up by Penn State geography prof Peter Gould-yields projections of the epidemic's growth and change that are fundamental to health care planning. "If you can say something sensible about how people move, you can say something sensible about how HIV moves," said Gould, whose "AIDS diffusion maps" have appeared everywhere from academic journals to Playboy. While his first maps tracked the epidemic up to 1990, this retroviral Jeanne Dixon is polishing some post-millenium prophesies.