The function f(x) takes a string input, moves the first and the second characters to the end of the string, and adds the digit '1' between them.

[CODE]:
```python
def approx_function(s: str) -> str:
    return s[2:] + s[0] + '1' + s[1]
```

Let's test the function with a few more inputs to confirm this pattern.

COMMAND: PYTHON(./temp/function.py "qwerty" "abcdef" "azerty" "function" "ixowqew")