Array ( [0] => Array ( [title] => L21A [link] => https://www.youtube.com/embed/PY8z4TTqXkw ) [1] => Array ( [title] => L21B [link] => https://www.youtube.com/embed/-BeFrE4Y1tY ) ) 國立清華大學開放式課程OpenCourseWare(NTHU, OCW) - Lecture 21 | Variation-of-constant formula for linear discrete DDEs