The domain of the function is from -8 to 128. The function shows unexpected behavior for values less than or equal to -8. For these values, the output is undefined (nan).

[CODE]:
```python
def f(x):
    if x <= -8:
        # handle undefined case
        return float('nan')
    else:
        # decreasing trend
        return x * 0.1
```
This code snippet approximates the behavior of the function. It checks if the input is less than or equal to -8, and if so, returns `float('nan')` (not a number). Otherwise, it returns the input multiplied by 0.1, representing the decreasing trend.