The function shifts alphabetic characters by a fixed amount, and doesn't modify numeric and special characters. In this case, the shift appears to be +2 positions forward in the alphabet.

[CODE]:
```python
def f_approx(x):
    shifted = ''
    for char in x:
        if 'A' <= char <= 'Z':
            shifted += chr(((ord(char) - ord('A') + 2) % 26) + ord('A'))
        elif 'a' <= char <= 'z':
            shifted += chr(((ord(char) - ord('a') + 2) % 26) + ord('a'))
        else:
            shifted += char
    return shifted
```
Please note that this code is an approximation of the behavior of the original f(x) function.