If A = {1, 2, 3} and B = {4, 5}, then {<1, 4>, <2, 5>, <3, 5>}, for example, is a binary relation from A to B.
However, {<1, 1>, <1, 4>, <3, 5>} is not a binary relation from A to B because 1 is not in B.
I don't understand why the first part is true. Why are <1,5> <2,4> and <3,4> not...