Brooklyn Youth, 15, Places No. 1 in Math Olympiad
15-yr-old Stuyvesant HS (NYC) student Paul Zeitz scores highest in US on Mathematical Olympiad, 5-problem test of mathematical reasoning and ingenuity; Stuyvesant students Eric Lander and Jesse Deutsch score 5th and 14th respectively; Zeitz and Lander will be among 8 US team members competing in World Mathematical Olympiad in Erfurt (E Ger); Rutgers Univ Prof Samuel L Greitzer, who ran contest, and Zeitz comment; sample from test; Zeitz illus (M)