The function f(x) takes a string as input, calculates the sum of the digits in the input, and returns a word corresponding to the sum.

[CODE]:
```python
def f_approx(x):
    digit_sum = sum(int(d) for d in x if d.isdigit())
    word_mapping = {
        0: 'zeso',
        1: 'zeso',
        2: 'zeso',
        3: 'tw',
        4: 'th',
        5: 'fi',
        6: 'tw',
        7: 'tw',
        8: 'th',
        9: 'tw'
    }
    return word_mapping.get(digit_sum, '')
```