DES algorithm exhaustive key search 2^55 instead of 2^56

Question

How is it possible to do an exhaustive key search (DES algorithm) and only search for 2^55 instead of 2^56 keys ?

This is a slide from my course if this helps you. I don't understand it.

Answer 1

What you want follows from the DES complementation property so only the remaining 55 key bits need to be brute-forced.