Question 751189
Elements in the domain are mapped to exactly one element in the codomain, therefore:

1.  (4,1)(3,2)(2,3)(1,8)  yes
2.  (1,3)(2,3)(3,3)(4,3)  yes
3.  (1,1)(2,2)(3,3)(4,4)  yes 
4.  (2,1)(4,2)(5,3)(-2,4) yes
5.  (8,1)(8,2)(8,3)(8,4)   no!
6.  (-2,-2)(0,0)(6,6)(-8,-8)  yes



:)