The function f(x) has the following behavior:
1. If the input starts with a lowercase letter, it swaps the first and second characters.
2. If the input starts with an uppercase letter and is followed by another letter, it replaces the second character with its next lower case character, while keeping the first character as is.
3. If the input starts with a number or a special character, it leaves the input unchanged.

[CODE]:
```python
def f(x):
    if x[0].islower() and len(x) > 1:
        return x[1] + x[0] + x[2:]
    elif x[0].isupper() and len(x) > 1 and x[1].isalpha():
        return x[0] + chr(ord(x[1]) + 1) + x[2:]
    else:
        return x
```