# Lower bounds for small depth arithmetic circuits Chandan Saha Joint work with Neeraj Kayal (MSRI) Nutan Limaye (IITB) Srikanth Srinivasan (IITB)

## Presentation on theme: "Lower bounds for small depth arithmetic circuits Chandan Saha Joint work with Neeraj Kayal (MSRI) Nutan Limaye (IITB) Srikanth Srinivasan (IITB)"— Presentation transcript:

Lower bounds for small depth arithmetic circuits Chandan Saha Joint work with Neeraj Kayal (MSRI) Nutan Limaye (IITB) Srikanth Srinivasan (IITB)

Arithmetic Circuit: A model of computation + xxxx ++++ xxxx …. ….. x1x1 x2x2 x n-1 xnxn f(x 1, x 2, …, x n )--> multivariate polynomial in x 1, …, x n x g h gh + g h g+h Product gate Sum gate There are `field constants’ on the wires

Arithmetic Circuit: A model of computation + xxxx ++++ xxxx …. ….. x1x1 x2x2 x n-1 xnxn f(x 1, x 2, …, x n ) Depth = 4

Arithmetic Circuit: A model of computation + xxxx ++++ xxxx …. ….. x1x1 x2x2 x n-1 xnxn f(x 1, x 2, …, x n ) Size = no. of gates and wires

The lower bound question Is there an explicit family of n-variate, poly(n) degree polynomials {f n } that requires… …super-polynomial in n circuit size ?

The lower bound question Is there an explicit family of n-variate, poly(n) degree polynomials {f n } that requires… …super-polynomial in n circuit size ? Note : A random polynomial has super-poly(n) circuit size

The Permanent – an explicit family Perm n = ∑ ∏ x i σ(i) σ є S n i є [n]

The Permanent – an explicit family Degree of Perm n is low. i.e. bounded by poly(n) Perm n = ∑ ∏ x i σ(i) σ є S n i є [n]

The Permanent – an explicit family Degree of Perm n is low. Coefficient of any given monomial can be found efficiently. …given a monomial, there’s a poly-time algorithm to determine the coefficient of the monomial. Perm n = ∑ ∏ x i σ(i) σ є S n i є [n]

The Permanent – an explicit family Degree of Perm n is low. Coefficient of any given monomial can be found efficiently. These two properties characterize explicitness Perm n = ∑ ∏ x i σ(i) σ є S n i є [n]

The Permanent – an explicit family Degree of Perm n is low. Coefficient of any given monomial can be found efficiently. Define class VNP Perm n = ∑ ∏ x i σ(i) σ є S n i є [n]

The Permanent – an explicit family Degree of Perm n is low. Coefficient of any given monomial can be found efficiently. Define class VNP Perm n = ∑ ∏ x i σ(i) σ є S n i є [n] Class VP: Contains families of low degree polynomials {f n } that can be computed by poly(n)-size circuits.

The Permanent – an explicit family Degree of Perm n is low. Coefficient of any given monomial can be found efficiently. Perm n = ∑ ∏ x i σ(i) σ є S n i є [n] VP vs VNP: Does Perm n family require super-poly(n) size circuits?

A strategy for proving arithmetic circuit lower bound Step 1: Depth reduction Step 2: Lower bound for small depth circuits

A strategy for proving arithmetic circuit lower bound Step 1: Depth reduction Step 2: Lower bound for small depth circuits

Notations and Terminologies Notations: n = no. of variables in f n d = degree bound on f n = n O(1) Homogeneous polynomial: A polynomial is homogeneous if all its monomials have the same degree (say, d). Homogeneous circuits: A circuit is homogeneous if every gate outputs/computes a homogeneous polynomial. Multilinear polynomial: In every monomial, degree of every variable is at most 1.

Reduction to depth ≈ log d Valiant, Skyum, Berkowitz, Rackoff (1983). Homogeneous, degree d, f n computed by poly(n) circuit f n computed by homogeneous poly(n) circuit of depth O(log d) arbitrary depth ≈ log d poly(n)

Reduction to depth 4 Agrawal, Vinay (2008); Koiran (2010); Tavenas (2013). Homogeneous, degree d, f n computed by poly(n) circuit f n computed by homogeneous depth 4 circuit of size n O(√d) ≈ log d 4 n O(√d) poly(n)

Reduction to depth 4 Agrawal, Vinay (2008); Koiran (2010); Tavenas (2013). Homogeneous, degree d, f n computed by poly(n) circuit f n computed by homogeneous depth 4 circuit of size n O(√d) ≈ log d 4 n O(√d) poly(n) … f n can have n O(d) monomials !

A depth 4 circuit + xxxx ++++ xxxx …. ….. x1x1 x2x2 x n-1 xnxn ∑ ∏ ∑ ∏

A depth 4 circuit + xxxx ++++ xxxx …. ….. x1x1 x2x2 x n-1 xnxn ∑ ∏ Q ij ij sum of monomials Q ij

Reduction to depth 3 Gupta, Kamath, Kayal, Saptharishi (2013); Tavenas (2013). Homogeneous, degree d, f n computed by poly(n) circuit f n computed by depth 3 circuit of size n O(√d) 3 n O(√d) 4

Reduction to depth 3 Gupta, Kamath, Kayal, Saptharishi (2013); Tavenas (2013). Homogeneous, degree d, f n computed by poly(n) circuit f n computed by depth 3 circuit of size n O(√d) 3 n O(√d) 4 not homogeneous!

A depth 3 circuit + xxxx ++++ …. x1x1 x2x2 x n-1 xnxn ∑ ∏ l ij ij linear polynomial l ij bottom fanin

Implication of the depth reductions Let {f n } be an explicit family of polynomials. if f n takes n ω(√d) size homogeneous if f n takes n ω(√d) size VP ≠ VNP or 4 3

A strategy for proving arithmetic circuit lower bound Step 1: Depth reduction Step 2: Lower bound for small depth circuits

Lower bound for homogeneous depth 4 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… …any homogeneous depth-4 circuit computing f n has size n Ω(√d) size = n Ω(√d) 4 fnfn

Lower bound for homogeneous depth 4 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… …any homogeneous depth-4 circuit computing f n has size n Ω(√d) size = n Ω(√d) 4 fnfn f n = i ∑ ∏ Q ij … has size n Ω(√d) j sum of monomials

Lower bound for homogeneous depth 4 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… …any homogeneous depth-4 circuit computing f n has size n Ω(√d) size = n Ω(√d) 4 fnfn …joint work with Kayal, Limaye, Srinivasan

Lower bound for homogeneous depth 4 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… …any homogeneous depth-4 circuit computing f n has size n Ω(√d) size = n Ω(√d) 4 fnfn …the technique appears to be using homogeneity crucially

Lower bound for depth 3 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… any depth-3 circuit (bottom fanin ≤ √d) computing f n has size n Ω(√d) size = n Ω(√d) 3 fnfn

Lower bound for depth 3 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… any depth-3 circuit (bottom fanin ≤ √d) computing f n has size n Ω(√d) size = n Ω(√d) 3 fnfn needn’t be homogeneous

Lower bound for depth 3 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… any depth-3 circuit (bottom fanin ≤ √d) computing f n has size n Ω(√d) size = n Ω(√d) 3 fnfn Note: Even for bottom fanin ≤ √d, depth-3 circuits n ω(√d) VP ≠ VNP

Lower bound for depth 3 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… any depth-3 circuit (bottom fanin ≤ t) computing f n has size n Ω(d/t) size = n Ω(d/t) 3 fnfn …joint work with Kayal

Lower bound for depth 3 Theorem: There is a family of homogeneous polynomials {f n } in VNP (with deg f n = d) such that… any depth-3 circuit (bottom fanin ≤ t) computing f n has size n Ω(d/t) size = n Ω(d/t) 3 fnfn … answers a question by Shpilka & Wigderson (1999)

Proof ideas

Homogeneous depth-4 lower bound

Complexity measure A measure is a function μ: F[x 1, …, x n ] -> R. We wish to find a measure μ such that 1.If C is a circuit (say, a depth 4 circuit) then μ(C) ≤ s. “small quantity”, where s = size(C) 2.For an “explicit” polynomial f n, μ(f n ) ≥ “large quantity” Implication: If C = f n then s ≥ “large quantity” “small quantity”  Upper bound  Lower bound

Some complexity measures Measure Model Partial derivatives (Nisan & Wigderson) homogeneous depth-3 circuits Evaluation dimension (Raz) multilinear formulas Hessian (Mignon & Ressayre) determinantal complexity permanent Jacobian (Agrawal et. al.) occur-k, depth-4 circuits Incomplete list ?

Some complexity measures Measure Model Partial derivatives (Nisan & Wigderson) homogeneous depth-3 circuits Evaluation dimension (Raz) multilinear formulas Hessian (Mignon & Ressayre) determinantal complexity permanent Jacobian (Agrawal et. al.) occur-k, depth-4 circuits Shifted partials (Kayal; Gupta et. al.) homog. depth-4 with low bottom fanin Projected shifted partials homogeneous depth-4 circuits; depth-3 circuits (with low bottom fanin)

Space of Partial Derivatives Notations: ∂ =k f : Set of all k th order derivatives of f(x 1, …, x n ) : The vector space spanned by F-linear combinations of polynomials in S Definition: PD k (f) = dim( ) Sub-additive property: PD k (f 1 + f 2 ) ≤ PD k (f 1 ) + PD k (f 2 )

Space of Shifted Partials Notation: x =ℓ = Set of all monomials of degree ℓ Definition: SP k,ℓ (f) := dim ( ) Sub-additivity: SP k,ℓ (f 1 + f 2 ) ≤ SP k,ℓ (f 1 ) + SP k,ℓ (f 2 )

Space of Shifted Partials Notation: x =ℓ = Set of all monomials of degree ℓ Definition: SP k,ℓ (f) := dim ( ) Sub-additivity: SP k,ℓ (f 1 + f 2 ) ≤ SP k,ℓ (f 1 ) + SP k,ℓ (f 2 ) Why do we expect SP(C) to be small ?

Shifted partials – the intuition C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Q ij = Sum of monomials

Shifted partials – the intuition C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Observation: ∂ =k Q i1 …Q im has “many roots” if k << m << n … any common root of Q i1 …Q im is also a common root of ∂ =k Q i1 …Q im

Shifted partials – the intuition C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Observation: Dimension of the variety of ∂ =k Q i1 …Q im is large if k << m << n

Shifted partials – the intuition C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Observation: Dimension of the variety of ∂ =k Q i1 …Q im is large if k << m << n [Hilbert’s] Theorem (informal): If dimension of the variety of {g} is large then dim ( ) is small.

Shifted partials – the intuition C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Observation: Dimension of the variety of ∂ =k Q i1 …Q im is large if k << m << n [Hilbert’s] Theorem (informal): If dimension of the variety of {g} is large then dim ( ) is small. … so we expect SP k,ℓ (Q i1 …Q im ) to be a `small quantity’

Shifted partials – the intuition C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Observation: Dimension of the variety of ∂ =k Q i1 …Q im is large if k << m << n [Hilbert’s] Theorem (informal): If dimension of the variety of {g} is large then dim ( ) is small. … by subadditivity, SP k,ℓ (C) ≤ s. `small quantity’

Depth-4 with low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Q ij = Sum of monomials of degree ≤ t (w.l.o.g m ≤ 2d/t )

Depth-4 with low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … degree ≤ k.t

Depth-4 with low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … at most ( ) terms m k

Depth-4 with low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … u. ∂ =k Q i1 …Q im = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … X degree = ℓ degree ≤ ℓ + k.t

Depth-4 with low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … u. ∂ =k Q i1 …Q im = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … X n + ℓ + kt n m k SP k,ℓ (Q i1 …Q im ) ≤ ( ). ( )

Depth-4 with low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … u. ∂ =k Q i1 …Q im = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … X n + ℓ + kt n m k SP k,ℓ (C) ≤ s. ( ). ( )  Upper bound

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm (homog. depth 4) Q ij = Sum of monomials (NO degree restriction)

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Idea: Reduce to the case of low bottom degree using Random restriction Multilinear projection

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Random restriction: Set every variable to zero independently at random with a certain probability. …denoted naturally by a map σ

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Random restriction: Set every variable to zero independently at random with a certain probability. …denoted naturally by a map σ σ(C) = σ(Q 11 ) σ(Q 12 )…σ(Q 1m ) + … + σ(Q s1 ) σ(Q s2 )…σ(Q sm ) Obs: If a monomial u has many variables (high support) then σ(u) = 0 w.h.p

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Random restriction: Set every variable to zero independently at random with a certain probability. …denoted naturally by a map σ σ(C) = σ(Q 11 ) σ(Q 12 )…σ(Q 1m ) + … + σ(Q s1 ) σ(Q s2 )…σ(Q sm ) w.l.o.g σ(Q ij ) = sum of ‘low support’ monomials

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Random restriction: Set every variable to zero independently at random with a certain probability. Homogeneous depth 4  homogenous depth 4 with low bottom support … w.l.o.g assume that C has low bottom support

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Projection map: π (g) = sum of the multilinear monomials in g

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Projection map: π (g) = sum of the multilinear monomials in g Observation: π (sum of ‘low support’ monomials) = sum of ‘low degree’ monomials

Reduction to low bottom degree C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Projection map: π (g) = sum of the multilinear monomials in g Observation: π (Q ij ) = sum of ‘low degree’ monomials

Projected Shifted Partials PSP k,ℓ (f) := dim (π (x =ℓ. ∂ =k f) ) (obeys subadditivity)

Projected Shifted Partials PSP k,ℓ (f) := dim (π (x =ℓ. ∂ =k f) ) (obeys subadditivity) multilinear shifts only!

Projected Shifted Partials PSP k,ℓ (f) := dim (π (x =ℓ. ∂ =k f) ) (obeys subadditivity) multilinear derivatives!

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm support of every monomial bounded by t

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Every variable in every monomial has degree 2 or less

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Every monomial has a variable with degree 3 or more

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Q i1 Q i2 …Q im = Q’ i1 Q’ i2 …Q’ im + Every monomial has a variable with degree 3 or more

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Q i1 Q i2 …Q im = Q’ i1 Q’ i2 …Q’ im + PSP k,ℓ (Q i1 Q i2 …Q im ) ≤ PSP k,ℓ (Q’ i1 Q’ i2 …Q’ im ) + PSP k,ℓ ( )

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Q i1 Q i2 …Q im = Q’ i1 Q’ i2 …Q’ im + PSP k,ℓ (Q i1 Q i2 …Q im ) ≤ PSP k,ℓ (Q’ i1 Q’ i2 …Q’ im ) + PSP k,ℓ ( ) 0

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Q i1 Q i2 …Q im = Q’ i1 Q’ i2 …Q’ im + PSP k,ℓ (Q i1 Q i2 …Q im ) ≤ PSP k,ℓ (Q’ i1 Q’ i2 …Q’ im ) + PSP k,ℓ ( ) 0 degree ≤ 2t

Depth-4 with low bottom support C = Q 11 Q 12 …Q 1m + … + Q s1 Q s2 …Q sm Q ij = Q’ ij + Q i1 Q i2 …Q im = Q’ i1 Q’ i2 …Q’ im + PSP k,ℓ (Q i1 Q i2 …Q im ) ≤ PSP k,ℓ (Q’ i1 Q’ i2 …Q’ im ) Abusing notation: Call Q’ ij as Q ij

Depth-4 with low bottom support ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … degree ≤ 2kt

Depth-4 with low bottom support ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … u. ∂ =k Q i1 …Q im = u. Q i k+1 … Q im + u. Q i1 Q i k+2 … Q im + X degree = ℓ degree ≤ 2kt

Depth-4 with low bottom support ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … π(u.∂ =k Q i1 …Q im ) = π( Q i k+1 … Q im ) + π( Q i1 Q i k+2 … Q im ) + X multilinear, degree ≤ ℓ + 2k.t

Depth-4 with low bottom support ∂ =k Q i1 …Q im = Q i1 Q i2 …Q ik …Q im + Q i1 Q i2 …Q ik Q i k+1 …Q im + … X...... = Q i k+1 … Q im + Q i1 Q i k+2 … Q im + … π(u.∂ =k Q i1 …Q im ) = π( Q i k+1 … Q im ) + π( Q i1 Q i k+2 … Q im ) + X  Upper bound ℓ + 2kt n m k SP k,ℓ (C) ≤ s. ( ). ( )

How large can PSP(f) be? Trivially, PSP k,ℓ (f) ≤ min { ( ).( ), ( ) } n k n ℓ n ℓ + d - k

How large can PSP(f) be? Trivially, PSP k,ℓ (f) ≤ min { ( ).( ), ( ) } n k n ℓ n ℓ + d - k Size of the set { x =ℓ. ∂ =k f } ≤ ( ).( ) Number of monomials in any polynomial in π (x =ℓ. ∂ =k f) ≤ ( ) n k n ℓ n ℓ + d - k Let f be a multilinear polynomial

How large can PSP(f) be? Trivially, PSP k,ℓ (f) ≤ min { ( ).( ), ( ) } Best lower bound for s s ≥ n k n ℓ n ℓ + d - k min {( ).( ), ( )} ( ).( ) m k n ℓ + 2kt n k n ℓ n ℓ + d - k = n Ω(d/t) After setting k and ℓ appropriately

How large can PSP(f) be? Trivially, PSP k,ℓ (f) ≤ min { ( ).( ), ( ) } Best lower bound for s s ≥ There’s an explicit f such that PSP k,ℓ (f) is close to the trivial upper bound.  (lower bound) n k n ℓ n ℓ + d - k min {( ).( ), ( )} ( ).( ) m k n ℓ + 2kt n k n ℓ n ℓ + d - k = n Ω(d/t)

Depth-3 lower bound

Trading depth for homogeneity Idea: Depth-3 with low bottom fanin Homogeneous depth-4 with low bottom support Size = s Bottom fanin = t 3 fnfn 4 (homogeneous) fnfn Size = s. 2 O(√d) Bottom support = t

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) C = α 1.(1 + l 11 )(1 + l 12 )…(1 + l 1m ) + …. + α s.(1 + l s1 )(1 + l s2 )…(1 + l sm ) linear forms field constants

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) C = (1 + l 11 )(1 + l 12 )…(1 + l 1m ) + …. + (1 + l s1 )(1 + l s2 )…(1 + l sm ) Notation: [g] d = d-th homogeneous part of g Easy observation: If C = f, which is homogeneous deg d polynomial, then [C] d = f.

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) C = (1 + l 11 )(1 + l 12 )…(1 + l 1m ) + …. + (1 + l s1 )(1 + l s2 )…(1 + l sm ) [C] d = [(1 + l 11 )(1 + l 12 )…(1 + l 1m )] d +….+ [(1 + l s1 )(1 + l s2 )…(1 + l sm )] d idea: transform these to homogeneous depth-4

Newton’s identities E d (y 1, y 2, …, y m ) := ∑ ∏ y j P r (y 1, y 2, …, y m ) := ∑ y j r S in 2 [m] |S| = d j in S (elementary symmetric polynomial of degree d) j in [m] (power symmetric polynomial of degree r)

Newton’s identities E d (y 1, y 2, …, y m ) := ∑ ∏ y j P r (y 1, y 2, …, y m ) := ∑ y j r S in 2 [m] |S| = d j in S j in [m] Lemma: E d (y) = ∑ β a ∏ P r (y) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar e.g. 2y 1 y 2 = (y 1 + y 2 ) 2 – y 1 2 – y 2 2 = P 1 2 – P 2 field constant

Newton’s identities E d (y 1, y 2, …, y m ) := ∑ ∏ y j P r (y 1, y 2, …, y m ) := ∑ y j r S in 2 [m] |S| = d j in S j in [m] Lemma: E d (y) = ∑ β a ∏ P r (y) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar Hardy-Ramanujan estimate: The number of a = (a 1, …, a d ) such that ∑ r.a r = d is 2 O(√d)

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) [(1 + l i1 )(1 + l i2 )…(1 + l im )] d = E d ( l i1, …, l im ) = ∑ β a ∏ P r ( l i1, …, l im ) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar 2 O(√d) summands

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) [(1 + l i1 )(1 + l i2 )…(1 + l im )] d = E d ( l i1, …, l im ) = ∑ β a ∏ P r ( l i1, …, l im ) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar 2 O(√d) summands Suppose every l ij has at most t variables, then…

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) [(1 + l i1 )(1 + l i2 )…(1 + l im )] d = E d ( l i1, …, l im ) = ∑ β a ∏ P r ( l i1, …, l im ) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar = ∑ β a ∏ Q i,a,r a = (a 1, …, a d ) ∑ r. a r = d r in [d] every monomial has support ≤ t

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) [(1 + l i1 )(1 + l i2 )…(1 + l im )] d = E d ( l i1, …, l im ) = ∑ β a ∏ P r ( l i1, …, l im ) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar = ∑ β a ∏ Q i,a,r a = (a 1, …, a d ) ∑ r. a r = d r in [d] [C] d = ∑ ∑ β a ∏ Q i,a,r a = (a 1, …, a d ) ∑ r. a r = d r in [d] i in [s]

Depth-3 to Depth-4 Implicit in Shpilka & Wigderson ; Hrubes & Yehudayoff (2011) [(1 + l i1 )(1 + l i2 )…(1 + l im )] d = E d ( l i1, …, l im ) = ∑ β a ∏ P r ( l i1, …, l im ) a = (a 1, …, a d ) ∑ r. a r = d r in [d] arar = ∑ β a ∏ Q i,a,r a = (a 1, …, a d ) ∑ r. a r = d r in [d] [C] d = ∑ ∑ β a ∏ Q i,a,r a = (a 1, …, a d ) ∑ r. a r = d r in [d] i in [s] Homogeneous depth-4 with low bottom support and size s.2 Ω(√d)

An explicit family with high PSP k,ℓ

An explicit family of polynomials Nisan-Wigderson family of polynomials: NW r := ∑ ∏ x i, h(i) d2d2 h(z) in F [z], deg(h) ≤ r i in [d] identifying the elements of F with {1,2, …, d 2 } d2d2

An explicit family of polynomials Nisan-Wigderson family of polynomials: NW r := ∑ ∏ x i, h(i) d2d2 h(z) in F [z], deg(h) ≤ r i in [d] `Disjointness’ property: Two monomials can share at most r ≈ d/3 variables. = + + … d r r d 2(r+1) monomials

Projected Shifted Partials of NW r The set π (x =ℓ. ∂ =k NW r ) has ( ).( ) elements. Every polynomial in π (x =ℓ. ∂ =k NW r ) is multilinear & homogeneous of degree (ℓ + d – k). n k n ℓ

Projected Shifted Partials of NW r The set π (x =ℓ. ∂ =k NW r ) has ( ).( ) elements. Every polynomial in π (x =ℓ. ∂ =k NW r ) is multilinear & homogeneous of degree (ℓ + d – k). PSP k,ℓ (NW r ) = rank (M) n k n ℓ M := ( ).( ) rows π (x =ℓ. ∂ =k NW r ) (0/1)-matrix of coefficients n ℓ + d - k ( ) columns n k n ℓ

Projected Shifted Partials of NW r Because of the `disjointness property’ of NW r, the columns of M are almost orthogonal. Hence, B := M T M is diagonally dominant. Observe, rank (M) ≥ rank (B).

Projected Shifted Partials of NW r Because of the `disjointness property’ of NW r, the columns of M are almost orthogonal. Hence, B := M T M is diagonally dominant. Observe, rank (M) ≥ rank (B). Alon’s rank bound (for diagonally dominant matrix): If B is a real symmetric matrix then rank (B) ≥ Tr (B) 2 Tr (B 2 )

Projected Shifted Partials of NW r [Main lemma]: Using Alon’s bound and settings r, k and ℓ appropriately, PSP k,ℓ (NW r ) ≥ η. min {( ).( ), ( )} n k n ℓ n ℓ + d - k small factor

An explicit family in VP [Kumar-Saraf (2014)] : Showed the same lower bound using the Iterated Matrix multiplication polynomial, which is in VP

An explicit family in VP [Kumar-Saraf (2014)] : Showed the same lower bound using the Iterated Matrix multiplication polynomial, which is in VP VNP Circuits (VP) ABPs Formulas Depth-4 exponential separation

An explicit family in VP [Kumar-Saraf (2014)] : Showed the same lower bound using the Iterated Matrix multiplication polynomial, which is in VP VNP Circuits (VP) ABPs Formulas Open: separation ? …known in the multilinear setting [Dvir, Malod, Perifel, Yehudayoff (2012)]

An explicit family in VP [Kumar-Saraf (2014)] : Showed the same lower bound using the Iterated Matrix multiplication polynomial, which is in VP VNP Circuits (VP) ABPs Formulas Open: separation ? …improve n Ω(√d) to n ω(√d)

Some other open questions 1.Prove a n Ω(√d) lower bound for general depth-3 circuits (i.e. without the low bottom fanin restriction).

Some other open questions 1.Prove a n Ω(√d) lower bound for general depth-3 circuits. 2.Prove a n Ω(√d) lower bound for homogeneous depth-5 circuits. [open problem in Nisan & Wigderson (1996)] (2)  (1)

Some other open questions 1.Prove a n Ω(√d) lower bound for general depth-3 circuits. 2.Prove a n Ω(√d) lower bound for homogeneous depth-5 circuits. 3.Prove a n Ω(d) lower bound for multilinear depth-3 circuits. (current best is 2 Ω(d) ) …interestingly, one can get this using PSP measure

Some other open questions 1.Prove a n Ω(√d) lower bound for general depth-3 circuits. 2.Prove a n Ω(√d) lower bound for homogeneous depth-5 circuits. 3.Prove a n Ω(d) lower bound for multilinear depth-3 circuits. 4.A separation between homogeneous formulas and homogeneous depth-4 formulas.

Some other open questions 1.Prove a n Ω(√d) lower bound for general depth-3 circuits. 2.Prove a n Ω(√d) lower bound for homogeneous depth-5 circuits. 3.Prove a n Ω(d) lower bound for multilinear depth-3 circuits. 4.A separation between homogeneous formulas and homogeneous depth-4 formulas. 5.A separation between homogeneous formulas and multilinear homogeneous formulas. …exhibiting the power of non-multilinearity

Some other open questions 1.Prove a n Ω(√d) lower bound for general depth-3 circuits. 2.Prove a n Ω(√d) lower bound for homogeneous depth-5 circuits. 3.Prove a n Ω(d) lower bound for multilinear depth-3 circuits. 4.A separation between homogeneous formulas and homogeneous depth-4 formulas. 5.A separation between homogeneous formulas and multilinear homogeneous formulas. Thanks!

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