The function takes a single lowercase English letter (from 'a' to 'l') as input and outputs the corresponding uppercase letter shifted by 15 positions in the alphabet. If the input string is not a lowercase English letter or has more than one character, the output will be an empty string.

[CODE]: 
```python
def f(x):
    if len(x) == 1 and 'a' <= x <= 'l':
        return chr(((ord(x) - ord('a') + 15) % 26) + ord('A'))
    else:
        return ""
```