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Putting a Junta to the Test
Putting a Junta to the Test
Joint work with Eldar Fischer, Dana Ron, Shmuel Safra, and Alex Samorodnitsky
Guy KindlerGuy Kindler
Property TestingProperty Testing
o P – propertyo f – input
o Goal: Distinguish, using the fewest possible queries, between• f has P• f is -far from having P
d(f,g) = Prx[f(x)≠g(x)]
d(f,g) = Prx[f(x)≠g(x)]
HistoryHistory
o Testing Proofs (PCP): BLR
o Combinatorial properties: GGR
o PRS: Logic AND, monotonous DNF.
JuntasJuntas
Boolean Functions:
nf : { 1,1} { 1,1}
f( )=
n entries
JuntasJuntas
Boolean Functions:
1 -1 1 1 1 1 -1 -1 1 -1 -1 1 1 -1 -1 -1f( )=-1
nf : { 1,1} { 1,1}
Juntas
1 -1 1 1 1 1 -1 -1 1 -1 -1 1 1 -1 -1 -1f( )=-1
j-junta: depends on at most j coordinates.
1 -1 1 1 1 1 -1 -1 1 -1 -1 1 1 -1 -1 -1
-1
JuntasJuntas
1 -1 1 1 1 1 -1 -1 1 -1 -1 1 1 -1 -1 -1f( )=-1-1 1 1 1 -1 1 1
j-junta: depends on at most j coordinates.
-1
1 -1 1 1 1 1 -1 -1 -1-1 1 1 1 -1 1 1f( )=
Definition of j-Junta TestDefinition of j-Junta Test
1 -1 1 1 1 1 -1 -1 1 -1 -1 1 1 -1 -1 -1
1 1 1 -1 -1 1 -1 1 1 1 -1 1 1 1 1 1
-1 1 1 -1 -1 -1 1 -1 1 -1 1 -1 1 -1 -1 1
1 -1 -1 -1 1 -1 1 -1 1 1 1 -1 1 -1 1 -1
-1 -1 -1 1 -1 1 1 -1 1 1 -1 1 -1 1 -1 -1
f( )=
f( )=
f( )=
f( )=
-1
1
1
1
-1
1 -1 1 1 1 1 -1 -1 -1-1 1 1 1 -1 1 1f( )=1 -1 1 1 1 1 -1 -1 1 -1 -1 1 1 -1 -1 -1
1 1 1 -1 -1 1 -1 1 1 1 -1 1 1 1 1 1
-1 1 1 -1 -1 -1 1 -1 1 -1 1 -1 1 -1 -1 1
1 -1 -1 -1 1 -1 1 -1 1 1 1 -1 1 -1 1 -1
-1 -1 -1 1 -1 1 1 -1 1 1 -1 1 -1 1 -1 -1
f( )=
f( )=
f( )=
f( )=
-1
1
1
1
-1
Accept?Accept?
Reject?Reject?
Definition of j-Junta TestDefinition of j-Junta Test
Before we test juntas…
Given a set I of coordinates, can we verify that f does not depend on it?
Given a set I of coordinates, can we verify that f does not depend on it?
I-independence testI-independence test
I-independence testI-independence test
I
-1 1 -1 -1 1 -1 1 1 1 -1 1
-1 1 -1 -1 1 -1 1 1 1 -1 1
I-independence testI-independence test
I
w
f( )=
f( )=
-1 1 -1 -1 1 -1 1 1 1 -1 1
-1 1 -1 -1 1 -1 1 1 1 -1 1 1 1 -1 -1 1
-1 1 1 -1 -1
-1
1
I-independence testI-independence test
I
f( )=
wf( )=
-1 1 -1 -1 1 -1 1 1 1 -1 1
-1 1 -1 -1 1 -1 1 1 1 -1 1 1 1 -1 -1 1
-1 1 1 -1 -1
-1
1
Claim: If Pr[I is detected] ≤ then f is at most ”-dependent on I” g independent of I, d(f,g)≤
variationf(I)
Claim: If Pr[I is detected] ≤ then f is at most ”-dependent on I”
I-independence testI-independence test
I
f( )=
wf( )=
-1 1 -1 -1 1 -1 1 1 1 -1 1
-1 1 -1 -1 1 -1 1 1 1 -1 1 1 1 -1 -1 1
-1 1 1 -1 -1
-1
1
g independent of I, d(f,g)≤
variationf(I)
Claim: If Pr[I is detected] ≤ then f is at most ”-dependent on I”
I-independence testI-independence test
Proof: Let g(w z0)=Majz{f(wz)}
Define p(w)=Prz[f(wz)≠g(w z)]
variationf(I)
dist(f,g)
wE p(w)
1 2w,z ,z 1 2 Pr [f(w z ) f(w z )]
wE 2p(w)(1 p(w))
I
f( )=
wf( )=
-1 1 -1 -1 1 -1 1 1 1 -1 1
-1 1 -1 -1 1 -1 1 1 1 -1 1 1 1 -1 -1 1
-1 1 1 -1 -1
-1
1
g independent of I, d(f,g)≤
variationf(I)
1 2w z ,z 1 2E Pr [f(w z ) f(w z )]
w,zPr f(w z) g(w z)
1. Partition the coordinates into r subsets.
The j-Junta TestThe j-Junta Test
I1 Ir
r=10j2r=10j2
A j-junta is independent of all but j subsets !
A j-junta is independent of all but j subsets !
1. Partition the coordinates into r subsets.
2. Run the independence-test r/ times on each subset.
The j-Junta TestThe j-Junta Test
I1 Ir
If f has variation “/r” on a subset, it is almost surely detected!
If f has variation “/r” on a subset, it is almost surely detected!
1. Partition the coordinates into r subsets.
2. Run the independence-test r/ times on each subset.
3. Accept if ≤j of the subsets are detected.
The j-Junta TestThe j-Junta Test
I1 Ir
Completeness:Completeness:
Soundness:
If f is far from being a junta,
then the test rejects with
probability ½.
Soundness:
If f is far from being a junta,
then the test rejects with
probability ½.
Lemma: For every Boolean f, unless
f is –close to a j-junta,
w.h.p.,
the test rejects.
SoundnessSoundness
at least j+1 subsets have variation /r
[n], ||≤j, variationf([n]\)<
over the partitions of [n],
VariationsVariations
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
11
VariationsVariations
I
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
26f̂ (S )
VariationsVariations
I
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
f Ivariation
We’ll prove that unless
f is –close to a j-junta,
w.h.p.
the test rejects.at least j+1 subsets have variation
/r
||≤j, and variationf([n]\)<
over the partitions of [n],
For t /r, let fi | i tvariationJ
I1 Ir
If ||>j, easy !!If ||>j, easy !!
We’ll prove that unless
f is –close to a j-junta,
w.h.p.
the test rejects.at least j+1 subsets have variation
/r
||≤j, and variationf([n]\)<
over the partitions of [n],
Fix t /r and let ii | f t variationJ
I1 Ir
Assume variationf([n]\)>.
Then !!!
Assume variationf([n]\)>.
Then !!!
mf[ (I )] / rE variation
Claim: w.h.p. mf (I \ ) / 2rvariation J
mi I ]=1/ r=J , Pr[o Recall: For each i in
Claim: w.h.p. mf (I \ ) / 2rvariation J
I
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
mi I ]=1/ r=J , Pr[o Recall: For each i in
Claim: w.h.p. mf (I \ ) / 2rvariation J
23f̂ (S )
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
22f̂ (S )
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
22f̂ (S )
I fu_variation
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
22f̂ (S )
I fu_variation
ff u_variation variationJ J
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
22f̂ (S )
I fu_variation
mI f / r E u_variation
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
22f̂ (S )
I fu_variation
mI with high probability !2r !f /variation
I
The Unique-VariationThe Unique-Variation
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
21f̂ (S )
22f̂ (S )
23f̂ (S )
24f̂ (S )
26f̂ (S )
27f̂ (S )
22f̂ (S )
I fu_variation
mI with high probability !2r !f /variation
Q.E.DQ.E.D
Other ResultsOther Results
o Shown number of queries: j4/o Using adaptivity: j3/o Using two-sidedness: j2/ o Allowing (2j)-juntas: j2/ o Variables in General probability
spaces.o “f” is “g” test, where g is a j-junta.o Lower Bound: at least (j)1/2 queries are
needed
Open ProblemsOpen Problems
o Improve lower bound to j2/(perhaps via random-walk convergence on Z2)
o “f is g” for non-juntas?
o Characterize efficiently testable properties via Fourier transform??