One of the primary goals of complexity theory is separating complexity classes, a.k.a proving lower bounds. Embarrassingly we have only a handful of unconditional separation results. Separating P from NP is of course the mother of all such goals. Anybody who understands the philosophical underpinnings of the P vs NP problem would love to LIVE to see its resolution. Towards resolving this, we made some (“anti”)-progress (Eg : Relativization, Natural proofs, Algebrization) and have a new geometric complexity theory approach which relies on an Extended-Extended-Extended-Extended-Riemann-Hypothesis !! For more information about the history and status of P vs NP problem read Sipser’s paper [Sipser'92], Allender’s status report [Allender'09] or Fortnow’s article [Fortnow'09].

Today’s post is about the **Logspace (L) vs Polynomial time (P)** problem, which (in my opinion) is right next to the P vs NP problem in its theoretical importance. I guess many researchers believe that . Did we make any progress/anti-progress towards resolving the conjecture ? Here are two attempts both based on branching programs and appeared in MFCS with a gap of 20 years !!

**1) A conjecture by Barrington and McKenzie (BM’89):** The problem is defined as follows :

: Given an table filled with entries from , which we interpret as the multiplication table of an -element groupoid, and a subset of which includes element 1, determine whether the subgroupoid , defined as the closure of under the groupoid product, includes element .

Barrington-McKenzie Conjecture :For each , a branching program in which each node can only evaluate a binary product within an -element groupoid, branching ways according to the possible outcomes, must have at least nodes to solve all instances with singleton starting set .

The problem is known to be P-complete [JL'76]. Barrington-McKenzie Conjecture would imply that for any . In particular, it would imply that . I don’t know if there is any partial progress towards resolving this conjecture.

**2) Thrifty Hypothesis :** This is a recent approach by Braverman et. al [BCMSW'09] towards proving a stronger theorem . Stephen Cook presented this approach at Valiant’s 60th birthday celebration and Barriers Workshop. He also announced a **$100 prize** for solving an intermediate open problem mentioned in his slides.

Tree Evaluation Problem (TEP):The input to the problem is a rooted, balanced -ary tree of height , whose internal nodes are labeled with -ary functions on , and whose leaves are labeled with elements of . Each node obtains a value in equal to its -ary function applied to the values of its children. The output is the value of the root.

Thrifty Hypothesis :Thrifty Branching Programs are optimal among deterministic branching programs solving TEP.

Open Problems :

- My knowledge about the history of L vs P problem is limited. Are there other approaches/attempts in the last four decades to separate L from P ?
- An intermediate open problem is mentioned in the last slide of these slides. The authors announced
$100 prizefor the first correct proof. Read their paper for more open problems.

*References :*

**[BM'89]**David A. Mix Barrington, Pierre McKenzie:**Oracle Branching Programs and Logspace versus P.***MFCS 1989: 370-379***[BCMSW'09]**Mark Braverman, Stephen A. Cook, Pierre McKenzie, Rahul Santhanam, Dustin Wehr:**Branching Programs for Tree Evaluation.***MFCS 2009: 175-186***[Sipser'92]**Michael Sipser:**The History and Status of the P versus NP Question**STOC 1992: 603-618**[Allender'09]**Eric Allender:**A Status Report on the P Versus NP Question.**Advances in Computers 77: 117-147 (2009)**[pdf]****[Fortnow'09]**Lance Fortnow:**The status of the P versus NP problem.**Commun. ACM 52(9): 78-86 (2009)**[pdf]****[JL'76]**Neil D. Jones, William T. Laaser:**Complete Problems for Deterministic Polynomial Time.***Theor. Comput. Sci. 3(1): 105-117 (1976)*

Hello Kintali

I haven’t solved that open problem despite many tries over the years. However a characterisation might interest you: L = NP iff read-only recursive programs are equivalent to read-only tail recursive programs, i.e., iff there is no need for the recursion stack in read-only computation.

An earlier first-order version in TCS:

http://portal.acm.org/citation.cfm?id=344414.319116

A later higher-order version in JFP:

http://portal.acm.org/citation.cfm?id=968494.968498

Regards and good luck!

Neil

Hi Neil. Thanks for the interesting links.

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Im working on a variation of the P vs L problem & came up with the following formulation. I believe it has deep connections to the theory at best and at least is a nice general framework for studying time/space tradeoffs, complexity class separations, etc.; if anyone agrees its a valid open problem please upvote my question, Id appreciate it.

http://cstheory.stackexchange.com/questions/9680/repetition-in-compressibility-of-tm-run-sequences

my original idea that led to this was a proof sketch along the lines that if P==L then P!=NP. I am still debating whether to write up this proof sketch somewhere in cyberspace (cstheory moderators/audience didnt react favorably so far)