Array ( [0] => Array ( [title] => L21A [link] => https://www.youtube.com/embed/jixtRvnft70 ) [1] => Array ( [title] => L21B [link] => https://www.youtube.com/embed/76X2i1vgR14 ) ) 國立清華大學開放式課程OpenCourseWare(NTHU, OCW) - 第21講 Number Theory