====== Lecture 7 ======
  * more about liminf and limsup. Ross 12
  * Series. basic definition and examples. partial sum. Cauchy conditions. absolute convergence and convergence. root test, ratio test, alternating series test. 

Discussion: 
  * Ross Ex 12.10, 12.12, 12.13
  * Ross Ex 14.1, 14.2, 14.10