The domain of the function is the set of all numerical values. However, there is an interval where the function displays unexpected behavior. For values less than or equal to 0, the function returns a positive value instead of the expected negative value.

[CODE]:
```python
def f(x):
    if x <= 0:
        return 4.1
    else:
        return -13
```
This code approximates the behavior of the function by returning 4.1 for any input less than or equal to 0, and -13 for any input greater than 0.