A sequence is a function whose domain is either all the integers between two given integers or all the integers greater than or equal to a given integer.
am, am + 1, am + 2, … , an,
ak (read “a sub k”) is called a term.
k in ak is called a subscript or index
am, am + 1, am + 2, … = infinite sequence
k = m에서 k=n까지의 summation의 표현
pi 기호를 사용해서 m부터 n까지의 곱을 표현한다.
n factorial denoted n! is defined to be the product of all the integers from 1 to n
n! = n(n-1)…32*1
Zero factorial 0! = 1