Given the relation \( R = \{(1, 3), (2, 3), (1, 4), (2, 5)\} \), determine if the relation is a function.

Step 1 ：A relation is a function if and only if each first element (input) is paired with exactly one second element (output).

Step 2 ：Looking at the given set, we see that the first element 1 is paired with 3 and also 4, and the first element 2 is paired with 3 and also 5.

Step 3 ：Therefore, since there are first elements paired with more than one second element, the given relation is not a function.