Computational Complexity


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Friday, October 24, 2003

When Good Theorems Have Bad Proofs

Here is one of my favorite examples of a bad proof for what turns out to be a correct theorem.

Theorem: If NP is in BPP then the whole polynomial-time hierarchy is in BPP.

Let's focus on simply showing Σ2 is in BPP if NP is in BPP. The rest is straightforward induction. Here is our first proof:

Do you see the problem with this proof?

To get a correct proof (first due to Zachos) we need to use Arthur Merlin games. Consider a Σ2 language L as an ∃∀ expression. Since NP is in BPP, we can replace the ∀ with a probabilistic test. This gives us what is known as MA or a Merlin-Arthur game where the powerful Merlin sends a message that Arthur can probabilistically verify. A now classic result shows that MA is contained in AM, where Arthur provides a random string to Merlin who must then provide a proof based on that string. Once again we apply the NP in BPP assumption to allow Arthur to simulate Merlin probabilistically and now we have a BPP algorithm for L.

The problem in the first proof is in the second "⊆". The assumption NP in BPP does not imply NPA in BPPA for all A.

8:59 AM # Comments []  

Wednesday, October 22, 2003

Talk to Me

How has the internet most affected the study of science? In one word: communication: the ability for scientists to discuss and share their research with each other quickly and cheaply. So I strive to find new ways to use the internet to improve communication. Starting this weblog is one such example. I'd thought I would try another: Instant Messaging.

Now many of you are thinking I am crazy, but for different reasons. Some of you out there have been using instant messaging for years and wondering how I could consider it s "new" technology. But many of you out there have barely figured out how to read your email attachments and have hardly even heard of IM.

On a trial basis, for my weblog readers, I will welcome your instant messages. Talk to me about this weblog, about complexity and computer science in general or about whatever you want. Maybe I'll start a trend and all computer scientists will IM each other. Maybe not but it's worth trying out.

I'm using Yahoo Instant Messaging; my Yahoo id is the imaginative "fortnow" (note: I do not read email sent to I put a button on the left column of the weblog home page that tells you when I am online and you can click to connect. I look forward to hearing from you.

10:23 AM # Comments []  

Monday, October 20, 2003

What's happening at the NSF?

There is a big reorganization in the CISE directorate of NSF. To understand what's happening, let's review the previous structure..

The National Science Foundation, like most government bureaucracies, has a tree-like structure. At top is the office of the director (Rita Colwell). Below that are several directorates including the Directorate for Computer and Information Science and Engineering (CISE) headed by Peter Freeman. By law every organization in NSF cannot be just "science" but "science and engineering" except for the Foundation itself.

Below CISE were several divisions, including Computer-Communications Research (C-CR) headed by Kamal Abdali. C-CR ha several programs including the Theory program headed by Ding-Zhu Du.

Peter Freeman, who recently became head of CISE, has decided to reorganize the whole directorate. Exactly what it will become should be announced next week but there are some hints in this presentation. Change is always scary but I'm hopeful theory will survive. I'll give more details when I know them.

To overcome the tree structure of NSF, there are a number of cross-disciplinary programs. One such program, Information and Technology Research (ITR), has produced several large, medium and small grants to a variety of projects, including many applications of theory. This is the last year of ITR solicitations and the calls have been well behind schedule, probably not unrelated to the CISE reorganization. This year's topic will be on "ITR for National Priorities" with more details promised by Thanksgiving. Unconfirmed rumors have the program will be more focused and only making medium sized grants.

6:41 PM # Comments []