Permanent and Determinant

Transcription

1 Permanent and Determinant non-identical twins Avi Wigderson IAS, Princeton

2 Meet the twins F field, char(f) 2. X M n (F) matrix of variables X ij Det n (X) = σ Sn sgn(σ) i [n] X iσ(i) Per n (X) = σ Sn i [n] X iσ(i) Homogeneous, multi-linear, degree n polynomials on n 2 variables, with 0,±1 coefficients.

3 Meet the twins Det n (X) Per n (X) σ Sn sgn(σ) i [n] X iσ(i) σ Sn i [n] X iσ(i) Physics: Fermions Bosons Knots: Alexander polynomial Jones polynomial Linear Algebra Enumeration /Counting Uses: Geometry / Volume Statistical Mechanics Everywhere Comput. Complexity Counting: Spanning trees Matchings Planar matchings Everything Complexity: Easy Hard (?) Boolean: NC-complete #P-complete Arithmetic: VP-complete VNP-complete

4 Complexity classes Permanent Hard Easy Determinant NP P Efficient proof/verification Efficient computation

5 Completeness [Valiant] Arithmetic VNP VP Permenent [Toda] Hard Easy Determinant Boolean EXP PSPACE Exponential time Polynomial space #P[Feynman] Counting BQP efficient quantum computation Bounded alternation PH NP P NC L Efficient proof/verification Efficient computation Fast parallel computation Logarithmic space

6 Arithmetic Computation Computing formal polynomials

7 Arithmetic complexity basics + + f c X 5 + F field + f n variables, deg f <n c + + X i X j Xi c Formula L(f) formula size X i X j X i c Circuit S(f) Circuit size Thm[VSBR]: S(f) L(f) S(f) logn

8 Complexity of Det Thm[Strassen]: S(Det n ) n 3 Thm[Csansky]: L(Det n ) n logn (no division!) (OPEN: poly?) Thm[Valiant]: If L(f)=s, then there is a 2s 2s matrix M f of vars and constants, f=det M f Proof: Induction 1 1 f=g+h M g M h M f f=g h M g M h M f = M g + M h M f = M g M h Determinantal representations of polynomials M f x M X

9 VNP completeness of Per Def[Valiant]: An integer polynomial f Z[X 1, X n ] is in VNP if each coefficient is efficiently computable. Intuitively, VNP captures all explicit polynomials! Thm[Valiant]: If f VNP, then there is a poly size matrix M f with f = Per M f Proof much more sophisticated

10 Algebraic analog of P NP Affine map L: M n (F) M k (F) is good if Per n = Det k L k(n): the smallest k for which there is a good map? a b c d [Polya] k(2) =2 Per 2 = Det 2 a b -c d [Valiant] k(n) < exp(n) [Mignon-Ressayre] k(n) > n 2 [Valiant] k(n) poly(n) VP VNP [Mulmuley-Sohoni] Geometric Complexity Theory (GCT): Per & Det are defined by their symmetries. Find, for k small, representation theoretic obstacles for good maps.

11 Arithmetic lower bounds for Det n & Per n Thm[Nisan] Both require non-commutative size 2 n arithmetic formulae. Open: l.b. for Circuits? Thm[Raz] Both require multi-linear arithmetic formulae of size n logn. Open: Exponential l.b.? Thm[Gupta-Kamath-Kayal-Saptharishi]: size 4 (Det n ) > n n size 4 (Per n ) > n n Tight!! Improvement VP VNP

12 Nice properties of Per & Complexity theoretic consequences

13 Nice properties of Per (and Det) (1) Downwards self-reducible Permanent of n n matrices efficiently computed from (several) permanents of smaller matrices. Row expansion Per n (X) = i [n] X 1i Per n-1 (X 1i )

14 Nice properties of Per (and Det) (2) Random self-reducible/correctible [Beaver-Feigenbaum, Lipton] The permanent of nxn matrices can be computed from the permanent of several random matrices. Assume C(Z)=Per n (Z) on 1/(8n) of Z M n (F) Interpolate Per n (X) on a random line: Y random, let g(t)=c(x+ty) a poly of degree n in t. M n (F) Eval on t=1,2,,n+1. C errs WHP g(t)=per(x+ty), x+y x so g(0)=per(x) x+3y x+2y

15 Hardness amplification If the Permanent can be efficiently computed for most inputs, then it can for all inputs! If the Permanent is hard in the worst-case, then it is also hard on average Worst-case Average case reduction Works for any low degree polynomial. Arithmetization: Boolean functions polynomials Lower bounds, derandomization, prob. proofs

16 Avalanche of consequences to probabilistic proof systems Using both RSR and DSR of Permanent! [Nisan] [Lund-Fortnow-Karloff-Nisan] [Shamir] Per 2IP Per IP IP = PSPACE [Babai-Fortnow-Lund] 2IP = NEXP [Arora-Safra, Arora-Lund-Motwani-Sudan-Szegedy] PCP = NP

17 Efficient Verification (skeptical, efficient) verifier vs. (untrusted, all powerful) Prover NP theorems with short written proofs sound & complete IP theorems with fast interactive proofs sound & complete WHP

18 Per IP [LFKN] How to check a theorem that has no short proof? Z i M i (F) a i F Verifier (untrusted) Prover Q n : what is Per(Z n )? A n : Per(Z n )= a n Q n-1 : what is Per(Z n-1 )? Q n-2 : what is Per(Z n-2 )? A n-1 : Per(Z n-1 )=a n-1 A n-2 : Per(Z n-2 )=a n-2 Q 2 : what is Per(Z 2 )? A 2 : Per(Z 2 )=a 2 Q 1 : what is Per(Z 1 )? A 1 : Per(Z 1 )=a 1 Claim: If A i is correct, than A i+1 is correct whp! Verifier can check Per(Z 1 )=a 1 without help.

19 A twist on Random-self-reducibility saw: compute one from many random inputs now: verify many from one random input Claims: Per(X 1 )=a 1,,Per(X k )=a k, X 1,,X k M n (F) Pick random X k+1, ask for g(t)=per(x t ), the unique deg k curve through X 1,,X k+1. Check for [1,k] Pick random r F, verify claim Per(X r )=g(r) M n (F) X 1 X 2 X i X k X k+1 X r

20 Boolean Computation Evaluating functions

21 The class #P (and P #P ) All natural counting problems. - # of sat assignments of a Boolean formula -# of cliques in a graph -# Hamilton cycles in a graph -# perfect matchings in a graph (Per) -# of linear extensions of a poset -# of spanning trees of a graph [Valiant] Decision Problem NP-complete in P ( Det [Kirchoff] ) #P # of accepting paths of an NP-machine. Knot Graph Statistical #P-complete problems Theory Theory Physics Evaluating Tutte, Jones, Chromatic, polynomials - # perfect matchings in planar gphs ( Det [Kasteleyn])

22 Quantum Computation BPP: Efficient probabilistic computation BQP: Efficient quantum computation Thm[Feynman, Bernstein-Vazirani] BQP P # P Thm[Shor] Factoring BQP (assumed not in BPP) -Can quantum computers be built? What can they do? Particles: Fermions (matter) Bosons (light, force) Wave function: Determinant Permanent [Valiant, Terhal-DiVincenzo, Knill] Fermionic computers = holographic algs Determinant [Aaronson-Arkhipov] Bosonic computers can sample the Permanent

23 Approximating Permanents of non-negative matrices

24 Approximating Per n [Valiant] Permanent of 0/1 matrices is #P-hard [Jerrum-Sinclair-Vigoda] Efficient probabilistic algorithm for (1+ε)-approximation for the permanent of any non-negative real matrix. Monte-Carlo Markov Chain (Glauber Dynamics, Metropolis algs, ) Such algs exist now for many #P-hard problems. Important interaction area for CS, Math, Physics

25 Approx Per n deterministically A: n n non-negative real matrix. [Linial-Samorodnitsky-Wigderson] Deterministic, efficient e n -factor approximation. Two ingredients: (1) [Falikman,Egorichev] If B Doubly Stochastic then e -n n!/n n Per(B) 1 (the lower bound solved van der Vaerden s conj) (2) Strongly polynomial algorithm for the following reduction to DS matrices: Matrix scaling: Find diagonal X,Y s.t. XAY is DS [Gurvits-Samorodnitsky 14] 2 n -factor approx. OPEN: Find a deterministic subexp approx.

26 Thanks!