L1_A
      Course Introduction
      Permutation and Combination
      Generating Functions
      Recurrence Relation
      Methods of Proving
      Number Theory
      Group Theory
      Automata Theory

L1_B
      References
      Intro to Combination Maths,CL Liu
      Concrete Maths, Graham, Knuth, Patashnik
      Proofs from the BOOK, Aigner, Ziegler

L1_C
      Permutation and Combination

L1_D
      Combinatorial Argument
      P(n,r)=P(r,r)*C(n,r)
      P(n,n)=P(n,r)*P(n-r,n-r)
      C(n.r)=C(n-1,r)＋C(n-1,r-1)
      P(n,r)=n!/(n-r)! ,C(n,r)=  n!/r!(n-r)!