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NP-hard Problems
• Vertex Cover• MAX-3SAT• Bin-Packing • Set Cover • Clique • MAX-CUT • ……………..• ……………..
Approximability : Algorithms
A C-approximation algorithm computes (C > 1), for problem instance I , solution A(I) s.t.
Minimization problems :
A(I) C OPT(I)
Maximization problems :
A(I) OPT(I) / C
Some Known Approximation Algorithms
• Vertex Cover 2 - approx.
• MAX-3SAT 8/7 - approx. Random assignment. • Packing/Scheduling (1+) – approx. > 0
(PTAS)
• Set Cover ln n approx.
• Clique n/log n [Boppana Halldorsson’92] • Many more , ref. [Vazirani’01]
PCP Theorem
[B’85, GMR’89, BFL’91, LFKN’92, S’92,……] [PY’91] [FGLSS’91, AS’92 ALMSS’92]
Theorem : It is NP-hard to tell whether a MAX-3SAT instance is * satisfiable (i.e. OPT = 1) or * no assignment satisfies more than 99%
clauses (i.e. OPT 0.99).
i.e. MAX-3SAT is 1/0.99 = 1.01 hard to approximate.
i.e. MAX-3SAT and MAX-SNP-complete problems [PY’91] have no PTAS.
Approximability : Towards Tight Hardness Results
• [Hastad’96] Clique n1-
• [Hastad’97] MAX-3SAT 8/7 -
• [Feige’98] Set Cover (1- ) ln n
[Dinur’05] Combinatorial Proof of PCP Theorem !
Open Problems in Approximability
– Vertex Cover (1.36 vs. 2) [DinurSafra’02]
– Coloring 3-colorable graphs (5 vs. n3/14) [KhannaLinialSafra’93, BlumKarger’97]– Sparsest Cut (1 vs. (logn)1/2) [AroraRaoVazirani’04]– Max Cut (17/16 vs 1/0.878… )
[Håstad’97, GoemansWilliamson’94] ………………………..
Unique Games Conjecture [Khot’02]
Implies these hardness results : • Vertex Cover 2- [KR’03]
• Coloring 3-colorable (1) [DMR’05]
graphs (variant of UGC)
• MAX-CUT 1/0.878.. - [KKMO’04]
• Sparsest Cut, Multi-cut [KV’05,
(1) CKKRS’04]
Min-2SAT-Deletion [K’02, CKKRS’04]
Unique Games Conjecture
Led to …
[MOO’05] Majority Is Stablest Theorem
[KV’05] “Negative type” metrics do not embed into L1 with O(1) “distortion”.
Optimal “integrality gap” for MAX-CUT
SDP with “Triangle Inequality”.
Integrality Gap : Definition Given : Maximization Problem + Specific SDP relaxation.
• For every problem instance G,
SDP(G) OPT(G)
• Integrality Gap = Max G SDP(G) / OPT(G)
• Constructing gap instance = negative result.
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
Unique Games Conjecture
• A maximization problem called “Unique Game” is hard to approximate.
• “Gap-preserving” reductions from Unique Game Hardness results for Vertex Cover,
MAX-CUT, Graph-Coloring, …..
Example of Unique GameOPT = max fraction of equations that can be satisfied by any assignment. x1 + x3 = 2 (mod k)
3 x5 - x2 = -1 (mod k)
x2 + 5 x1 = 0 (mod k)
UGC For large k, it is NP-hard to tell whether OPT 99% or OPT 1%
2-Prover-1-Round Game (Constraint Satisfaction
Problem )
variables
k labelsHere k=4
Constraints = Bipartite graphsor Relations [k] [k]
2-Prover-1-Round Game (Constraint Satisfaction
Problem )
variables
k labelsHere k=4
OPT(G) = 7/7
Find a labeling that satisfies max # constraints
Hardness of Finding OPT(G)
• Given a 2P1R game G, how hard is it to find OPT(G) ? • PCP Theorem + Raz’s Parallel Repetition Theorem
:
For every , there is integer k(), s.t. it is NP-hard to tell whether a 2P1R game with k = k() labels has OPT = 1 or OPT
In fact k = 1/poly()
Reductions from 2P1R Game
• Almost all known hardness results (e.g. Clique, MAX-3SAT, Set Cover, SVP, …. ) are reductions from 2P1R games.
• Many special cases of 2P1R games are known to be hard, e.g. Multipartite graphs,
Expander graphs, Smoothness property, ….
What about unique games ?
Unique Game = 2P1R Game with Permutations
variable
k labelsHere k=4
Permutations or matchings : [k] [k]
OPT(G) = 6/7
Find a labeling that satisfies max # constraints
Unique Game = 2P1R Game with Permutations
Unique Games Considered before …… [Feige Lovasz’92] Parallel Repetition of UG reduces OPT(G).
How hard is approximating OPT(G) for a unique game G ?
Observation : Easy to decide whether OPT(G) = 1.
MAX-CUT is Special Case of Unique Game
• Vertices : Binary variables x, y, z, w, …….
• Edges : Equations x + y = 1 (mod 2)
• [Hastad’97] NP-hard to tell whether OPT(MAX-CUT) 17/21 or OPT(MAX-CUT) 16/21
Unique Games Conjecture
For any , , there is integer k(, ), s.t. it is NP-hard to tell whether a UniqueGame with k = k(, ) labels has OPT 1- or OPT
i.e. Gap-Unique Game (1- , ) is NP-hard.
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
Case Study : MAX-CUT
• Given a graph, find a cut that maximizes fraction of edges cut.
• Random cut : 2-approximation.
• [GW’94] SDP-relaxation and rounding. min 0 < < 1 / (arccos (1-2) / ) = 1/0.878 … approximation.
• [KKMO’04] Assuming UGC, MAX-CUT is 1/0.878… - hard to approximate.
Reduction to MAX-CUT Unique Game Graph H
• Completeness : OPT(UG) > 1-o(1) - o(1) cut.
• Soundness : OPT(UG) < o(1) No cut with size arccos (1-2) / + o(1)
• Hardness factor = / (arccos (1-2) / ) - o(1)
• Choose best to get 1/0.878 … (= [GW’94])
Reduction from Unique Game
Gadget constructed via Fourier theorem + Connecting gadgets via Unique Game instance
[DMR’05] “UGC reduces the analysis of the entire construction to the analysis of the gadget”.
Gadget = Basic gadget ---> Bipartite gadget ---> Bipartite gadget with permutation
Basic Gadget
A graph on {0,1} k with specific properties
(e.g. cuts, vertex covers, colorability)
{0,1} k
k = # labelsx = 011
Y = 110
Basic Gadget : MAX-CUT Weighted graph, total edge weight = 1. Picking random edge : x R {0,1} k
y <-- flip every co-ordinate of x with
probability ( 0.8)
x
{0,1} k
y
MAX-CUT Gadget : Co-ordinate Cut Along Dimension i
Fraction of edges cut = Pr(x,y) [xi yi ]
=
Observation : These are the maximum cuts.
xi = 0 xi = 1
Bipartite Gadget with Permutation : [k] -> [k] Co-ordinates in second hypercube permuted via
.
x = 011
Y ’ = 110
(y’) = 011
Example : = reversal of co-ordinates.
Completeness : OPT(UG) 1-o(1)
label = 2
label = 1
label = 3
label = 1label = 1
label = 3
label = 2Labels = [1,2,3]
Completeness : OPT(UG) 1-o(1)
{0,1} k
Vertices
Edges
Hypercubes are cut along dimensions = labels.
MAX-CUT - o(1)
Proving Soundness
Unique Game Graph H
(Soundness) : OPT(UG) < o(1) H has no cut of size arccos (1-2) / + o(1)
MAX-CUT Gadget
Cuts = Boolean functions f : {0,1} k {0,1}
Compare boolean functions * that depend only on single co-ordinate
vs * where every co-ordinate has negligible “influence” (i.e. “non-junta” functions)
{0,1} k
x
y
f(x1 x2 …….. xk) = xi
f(x1 x2 …….. xk) = MAJORITY Influence (i, f) = Prx [ f(x) f(x+ei) ]
Gadget : “Non-junta” Cuts
How large can non-junta cuts be ? i.e. cuts with all influences negligible ? Random Cut : ½ Majority Cut : arccos (1-2) / > ½
• [MOO’05] Majority Is Stablest (Best) Any cut slightly better than Majority Cut must have “influential” co-ordinate.
Non-junta Cuts in Bipartite Gadget
[MOO’05] Any “special” cut with value arccos (1-2) / + must define a matching pair of influential co-ordinates.
{0,1} k {0,1} k
Non-junta Cuts in Bipartite Gadget
{0,1} k {0,1} k
f : {0,1} k --> {0, 1}
g : {0,1} k --> {0, 1}
i Infl (i, f), Infl (i, g) > (1)
cut > arccos (1-2) / +
Proving Soundness
• Assume arccos (1-2) / + cut exists.
• On /2 fraction of constraints, the bipartite gadget has arccos (1-2) / + /2 cut.
matching pair of labels on this constraint.
This is impossible since OPT(UG) = o(1).
Done !
Other Hardness Results• Vertex Cover Friedgut’s Theorem Every boolean function with low “average sensitivity” is a junta.
• Sparsest Cut, Min-2SAT Deletion KahnKalaiLinial Every balanced boolean function has a
co-ordinate with influence log n/n.
Bourgain’s Theorem (inspired by Hastad-Sudan’s 2-bit Long Code test)
Every boolean function with low “noise sensitivity” is a junta.
• Coloring 3-Colorable [MOO’05] inspired. Graphs
Basic Paradigm by [BGS’95,
Hastad’97] Hardness results for Clique, MAX-3SAT, ……. • Instead of Unique Games, use reduction from general 2P1R Games (PCP Theorem + Raz).
• Hypercube = Bits in the Long Code [Bellare
Goldreich Sudan’95]
• PCPs with 3 or more queries (testing Long Code).
• Not enough to construct 2-query PCPs.
Why UGC and not 2P1R Games?
Power in simplicity. “Obvious” way of encoding a
permutation constraint. Basic Gadget ----> Bipartite Gadget with permutation.
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
I Hope UGC is True• Implies all the “right” hardness results in a unifying way.
• Neat applications of Fourier theorems [Bourgain’02, KKL’88, Friedgut’98, MOO’05]
• Surprising application to theory of metric embeddings and SDP-relaxations [KV’05].
• Mere coincidence ?
Supporting Evidence
[Feige Reichman’04] Gap-Unique Game (C, ) is NP-hard.
i.e. For every constant C, there is s.t. it is NP-hard to tell if a UG has OPT > C or OPT < .
However C --> 0 as --> 0.
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
Disproving UGC means ..
For small enough (constant) , given a UG with optimum 1- , algorithm that finds a labeling
satisfying (say) 50% constraints.
Algorithmic Results
Algorithm that finds a labeling satisfying f(, k, n) fraction of constraints.
[Khot’02] 1- 1/5 k2 [Trevisan’05] 1- 1/3 log1/3 n [Gupta Talwar’05] 1- log n [CMM’05] 1/k , 1- 1/2 log1/2 k
None of these disproves UGC.
Quadratic Integer Program For Unique Game [Feige
Lovasz’92] variable
k labels
: [k] [k]
u1 , u2 , … , uk {0,1}
v1 , v2 , … , vk {0,1}
u
v
vi = 1 if Label(v) = i = 0 otherwise
Quadratic Program for Unique Games
Constraints on edge-set E.
• Maximize ui vπ(i)
(u, v) E i=1,2,..,k
• u i [k], ui {0,1}
• u ui2 = 1
i
• u i ≠ j , ui uj = 0
SDP Relaxation for Unique Games
• Maximize ui, vπ(i)
(u, v) E i=1,2,..,k
• u i [k], ui is a vector.
• u || ui ||2 = 1 i=1,2,..,k
• u i≠j [k], ui, uj = 0
[Feige Lovasz’92]
• OPT(G) SDP(G) 1.
• If OPT(G) < 1, then SDP(G) < 1.
• SDP(Gm) = (SDP(G))m
• Parallel Repetition Theorem for UG : OPT(G) < 1 OPT(Gm) 0
[Khot’02] Rounding Algorithm
u1
uk
u2
vk
v2
v1
r r
Label(u) = 2, Label(v) = 2
Pr [ Label(u) = Label(v) ] > 1 - 1/5 k2
Labeling satisfies 1 - 1/5 k2 fraction of constraints in expected sense.
Random ru v
[CMM’05] Algorithm• Labeling that satisfies 1/k fraction
of constraints. (Optimal [KV’05]) vk
v2
v1
r
u1
uk
u2
r
All i s.t. ui is “close” to r are taken as candidate labels to u.
Pick one of them at random.
[Trevisan’05] Algorithm
• Given a unique game with optimum 1- 1/log n, algorithm finds a labeling that satisfies 50% of constraints.
• Limit on hardness factors achievable via UGC (e.g. loglog n for Sparsest
Cut).
[Trevisan’05] Algorithm
[Leighton Rao’88] Delete a few constraints and
remaining graph has connected
components of low diameter.
Variables and constraints
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
[KV’05] Integrality Gaps for
SDP-relaxations • MAX-CUT • Sparsest Cut • Unique Game
Gaps hold for SDPs with “Triangle Inequality”.
Integer Program for MAX-CUT
Given G(V,E)
• Maximize ¼ |vi - vj |2
(i, j) E
• i, vi {-1,1}
• Triangle Inequality (Optional) : i, j , k, |vi - vj |2
+ |vj - vk |2 |vi - v k|2
Goemans-Williamson’s SDP Relaxation for MAX-CUT
• Maximize ¼ || vi - vj ||2
(i, j) E
• i, vi Rn, || vi || = 1
• Triangle Inequality (Optional) : i, j , k, || vi - vj ||2
+ || vj - vk ||2 || vi - v k||2
Integrality Gap for MAX-CUT• [Goemans Williamson’94]
Integrality gap 1/0.878..
• [Karloff’99] [Feige Schetchman ’01]
Integrality gap 1/0.878.. -
SDP solution does not satisfy Triangle Inequality.
Does Triangle Inequality make the SDP tighter ? NO if Unique Games Conj. is true !
Integrality Gap for Unique Games SDP
Unique Game G with
OPT(G) = o(1)
SDP(G) = 1-o(1)
OrthonormalBases for Rk
u1 , u2 ,
… , uk
v1 , v2 ,
… , vkvariables
k labels
Matchings [k] [k]u
v
Integrality Gap for MAX-CUT with
Triangle Inequality
{-1,1}k
u1 , u2 ,
… , uk
u1 u2 u3 ……… uk-1
uk
PCP Reduction
OPT(G) = o(1)
No large cut
Good SDP solution
Overview of the talk
• The UGC • Hardness of Approximation Results• I hope UGC is true • Attempts to Disprove : Algorithms
Connections/applications : • Fourier Analysis • Integrality Gaps• Metric Embeddings
Metrics and Embeddings
• Metric is a distance function on [n] such that
d(i, j) + d(j, k) d(i, k).
• Metric d embeds into metric with distortion 1 if i, j d(i, j) (i, j) d(i, j).
Negative Type Metrics
Given a set of vectors satisfying Triangle Inequality : i, j , k, || vi - vj ||2
+ || vj - vk ||2 || vi - v k||2
d(i, j) = || vi - vj ||2 defines a metric.
These are called “negative type metrics”.
L1 NEG METRICS
NEG vs L1 Question [Goemans, Linial’ 95] Conjecture : NEG metrics embed into
L1
with O(1) distortion.
Sparsest Cut
O(1) Integrality Gap O(1) Approximation
[Linial London Rabinovich’94][Aumann Rabani’98]
Unique Games Conjecture
[Chawla Krauthgamer Kumar Rabani Sivakumar ’05][KV’05]
(1) hardness result
NEG vs L1 Lower Bound
(loglog n) integrality gap for Sparsest
Cut SDP. [KhotVishnoi’05, KrauthgamerRabani’05]
A negative type metric that needs distortion (loglog n) to embed into L1.
Open Problems
• (Dis)Prove Unique Games Conjecture.
• Prove hardness results bypassing UGC.
• NEG vs L1 , Close the gap.
(log log n) vs (log n loglog n) [Arora Lee Naor’04]
Open Problems
• Prove hardness of Min-Deletion version of Unique Games. (log n approx. [GT’05])
• Integrality gaps with “k-gonal” inequalities.
• Is hypercube (Long Code) necessary ?
Open Problems More hardness results, integrality gaps, embedding lower bounds, Fourier Analysis,
……
[Samorodnitsky Trevisan’05] “Gowers Uniformity, Influence of Variables, and PCPs”. UGC Boolean k-CSP is hard to approximate within 2k- log k
Independent Set on degree D graphs is hard to approximate within D/poly(log D).
Open Problems in Approximability Traveling Salesperson
Steiner Tree Max Acyclic Subgraph, Feedback Arc Set Bin-packing (additive approximation) ……………………
Recent progress on Edge Disjoint Paths Network Congestion Shortest Vector Problem Asymmetric k-center (log* n) Group Steiner Tree (log2 n) Hypergraph Vertex Cover ………………
Linear Unique Games System of linear equations mod k. x1 + x3 = 2
3 x5 - x2 = -1
x2 + 5 x1 = 0
[KKMO’04] UGC UGC in the special case of linear equations mod k.