The function f(x) returns the input string as output if it consists of only letters, only numbers, or only special characters. However, if the input has a combination of letters, numbers, and special characters, it results in an error.

[CODE]:
```python
def approx_f(x):
    if x.isalpha() or x.isnumeric() or all(not c.isalnum() for c in x):
        return x
    else:
        return "Error: Combination of letters, numbers, and special characters not supported"
```
This approximating code will output the same result as the function for most inputs. Note that there could still be differences we haven't discovered yet.