The function maps each character of the input string to its corresponding number representation.

[CODE]:
```python
def f(x):
    number_mapping = {
        'a': 'one',
        'b': 'two',
        'c': 'three',
        'd': 'four',
        'e': 'five',
        'f': 'six',
        'g': 'seven',
        'h': 'eight',
        'i': 'nine',
        'j': 'ten',
        'k': 'eleven',
        'l': 'twelve',
        'm': 'thirteen',
        'n': 'fourteen',
        'o': 'fifteen',
        'p': 'sixteen',
        'q': 'seventeen',
        'r': 'eighteen',
        's': 'nineteen',
        't': 'twenty',
        'u': 'twenty-one',
        'v': 'twenty-two',
        'w': 'twenty-three',
        'x': 'twenty-four',
        'y': 'twenty-five',
        'z': 'twenty-six'
    }
    result = ""
    for char in x:
        if char.isalpha() and char.lower() in number_mapping:
            result += number_mapping[char.lower()]
    return result
```

Please note that this is an approximation based on the given experiment. To get a more accurate understanding of the function, it would be helpful to run experiments with a wider range of inputs.