Can someone give me the name of a paper where this is proved or maybe just prove it here (if it's easy enough)?
closed as off-topic by Emil Jeřábek, Sasho Nikolov, Marzio De Biasi, Hsien-Chih Chang 張顯之, Aryeh Nov 7 '17 at 21:47
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "Your question does not appear to be a research-level question in theoretical computer science. For more information about the scope, please see help center. Your question might be suitable for Computer Science which has a broader scope." – Emil Jeřábek, Sasho Nikolov, Marzio De Biasi, Hsien-Chih Chang 張顯之, Aryeh