 One month later he enrolled in a differential equations course ( " mostly full of seniors " ) as well . When Einar Hille temporarily replaced the regular instructor , Gleason found Hille 's style " unbelievably different ... He had a view of mathematics that was just vastly different ... That was a very important experience for me . So after that I took a lot of courses from Hille " including , in his sophomore year , graduate @-@ level real analysis . " Starting with that course with Hille , I began to have some sense of what mathematics is about . " 
