Any Math Majors? Need discrete mathamatics help..

[ifoam]

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 included in the relation?

I DO understand the second part. Since a binary relation from a set A to a set B is a set of ordered pairs <a, b> where a is an element of A and b is an element of B.

Anyone have any insite?

[GeorgePaul]

Originally Posted by ifoam

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 included in the relation?

I DO understand the second part. Since a binary relation from a set A to a set B is a set of ordered pairs <a, b> where a is an element of A and b is an element of B.

Anyone have any insite?

<1,5> <2,4> and <3,4> are not included in the relation because that's the way the relation is defined. <1,5> <2,4> and <3,4> could be included, but that would be a different relation. By the way, the relation as defined is a function from A to B.

[ifoam]

What if the relation isn't defined? That's all the information that i'm giving. how am i supposd to know that, that's the relation?

http://www.cs.odu.edu/%7Etoida/nerzi...efinition.html

The example is given at almost the end of the page