The function f(x) implements a Caesar cipher with a forward shift of 16.

[CODE]: 
```python
def caesar_cipher(input_string, shift):
    output_string = ""
    
    for char in input_string:
        if char.isalpha():
            is_upper = char.isupper()
            char = chr((((ord(char) - 65) + shift) % 26) + 65)
            if is_upper:
                output_string += char.upper()
            else: 
                output_string += char.lower()
        else: 
            output_string += char
            
    return output_string

input_string = "a"
shift = 16
print(caesar_cipher(input_string, shift))
```
Try this code to approximate the function's behavior.