In “The Julia Set,” she portrays a numbers genius in the most convincing way possible — by wearing her virtuosity lightly and casually, as if it were second nature (which, for someone like Julia, it is). But she also makes Julia a highly focused and mature young woman who is still, by virtue of age and inexperience, just naïve enough to be poised between assertion and following the lead of a dude she should have steered clear of.