The function shifts alphabetic characters forward by a fixed number of positions while keeping numeric characters unchanged.

[CODE]: 
```python
def f_approx(x):
    shift = 6
    shifted = []
    for c in x:
        if c.isalpha():
            if c.islower():
                shifted.append(chr(((ord(c) - ord('a') + shift) % 26) + ord('a')))
            else:
                shifted.append(chr(((ord(c) - ord('A') + shift) % 26) + ord('A')))
        else:
            shifted.append(c)
    return ''.join(shifted)
```