State the name of research postgraduate student among Professor Zhihua Zhou's research assistants.
research postgraduate student refers to type = 'RPG'; research assistant refers to the student who serves for research where the abbreviation is RA;
SELECT T3.f_name, T3.l_name FROM prof AS T1 INNER JOIN RA AS T2 ON T1.prof_id = T2.prof_id INNER JOIN student AS T3 ON T2.student_id = T3.student_id WHERE T1.first_name = 'Zhihua' AND T3.type = 'RPG' AND T1.last_name = 'Zhou'
database name:db_id cs_semester
course table course Cols: course id dtype integer, name dtype text, credit dtype integer, difficulty dtype integer, 
prof table prof Cols: professor id dtype integer, gender dtype text, first name dtype text, last name dtype text, email dtype text, popularity dtype integer, teaching ability dtype integer, graduate from dtype text, 
RA table RA Cols: student id dtype integer, capability dtype integer, professor id dtype integer, salary dtype text, 
registration table registration Cols: course id dtype integer, student id dtype integer, grade dtype text, satisfying degree dtype integer, 
student table student Cols: student id dtype integer, first name dtype text, last name dtype text, phone number dtype text, email dtype text, intelligence dtype integer, graduate point average dtype real, type dtype text, 
