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 prize for the first correct proof. Read their paper for more open problems.
- [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)