The function shifts each letter in the input string by a certain number of positions. It seems to apply a Caesar cipher to the letters. It does not affect numbers, spaces, or special characters.

[CODE]: 
```python
def f_approx(x):
    shifted_string = ""

    for char in x:
        if char.isalpha():
            shift = 20
            if char.islower():
                shifted_string += chr(((ord(char) - ord('a') + shift) % 26) + ord('a'))
            elif char.isupper():
                shifted_string += chr(((ord(char) - ord('A') + shift) % 26) + ord('A'))
        else:
            shifted_string += char

    return shifted_string
```

Please note that this code is an approximation of the original function and may not be an exact match.