The function takes an input string and returns a string of the alphabetic index values of each character (1-indexed) with "zero" added to the beginning. If a character is not alphabetic, it is ignored. The function is case-insensitive.

[CODE]: 
```python
def f(x):
    output = []
    x = x.lower()
    for char in x:
        if char.isalpha():
            index = ord(char) - ord('a') + 1
            output.append(str(index))
    return "zero " + " ".join(output)
```