The function appears to return the word for x+1 where x is the ASCII value of the input character. If the input is a digit, it returns 'aero'. If the input is a space, it also returns 'aero'.

[CODE]:
```python
def approx_function(input_string):
    if input_string.isalpha():
        return chr(ord(input_string) + 1)
    elif input_string == " ":
        return "aero"
    elif input_string.isdigit():
        return "aero"
    else:
        return "Error"
```