The function shifts lowercase and uppercase letters by 21 positions in the alphabet, wrapping around when necessary. It leaves numbers unchanged. 

[CODE]: 
def f(x):
    result = []
    for char in x:
        if char.isalpha():
            shift = 21
            origin = ord('A') if char.isupper() else ord('a')
            result.append(chr(((ord(char) - origin + shift) % 26) + origin))
        else:
            result.append(char)
    return ''.join(result)