The function `f(x)` takes an input string and modifies it by appending the letter "R" to the beginning and repeating the first character twice at the end.

[CODE]:
```python
def f(x):
    modified_string = "R" + x
    modified_string += x[0] * 2
    return modified_string
```

This code will produce a modified string that closely resembles the output of the `f(x)` function for a given input string.