The KRW conjecture Results and Open problems Or Meir
Introduction 1 Known results 2 Proof strategy 3 Future directions 4
Depth complexity Let f : { 0 , 1 } n → { 0 , 1 } . The depth complexity D ( f ) is the depth of the shallowest circuit for f . Captures the complexity of parallel computation.
Depth complexity Let f : { 0 , 1 } n → { 0 , 1 } . The depth complexity D ( f ) is the depth of the shallowest circuit for f . Captures the complexity of parallel computation. We only consider circuits with fan-in 2 .
Depth complexity Let f : { 0 , 1 } n → { 0 , 1 } . The depth complexity D ( f ) is the depth of the shallowest circuit for f . Captures the complexity of parallel computation. We only consider circuits with fan-in 2 . Major frontier: Explicit f with D ( f ) = ω (log n ) . a.k.a. P � = NC 1 .
Composition [Karchmer-Raz-Wigderson-91]: We need to understand composition.
Composition [Karchmer-Raz-Wigderson-91]: We need to understand composition. Let f : { 0 , 1 } m → { 0 , 1 } , g : { 0 , 1 } n → { 0 , 1 } . The composition f ⋄ g : { 0 , 1 } m × n → { 0 , 1 } is n g f . m a ( f ⋄ g )( X ) . X .
Composition [Karchmer-Raz-Wigderson-91]: We need to understand composition. Let f : { 0 , 1 } m → { 0 , 1 } , g : { 0 , 1 } n → { 0 , 1 } . The composition f ⋄ g : { 0 , 1 } m × n → { 0 , 1 } is n g f . m a ( f ⋄ g )( X ) . X .
Composition [Karchmer-Raz-Wigderson-91]: We need to understand composition. Let f : { 0 , 1 } m → { 0 , 1 } , g : { 0 , 1 } n → { 0 , 1 } . The composition f ⋄ g : { 0 , 1 } m × n → { 0 , 1 } is n g f . m a ( f ⋄ g )( X ) . X .
Composition [Karchmer-Raz-Wigderson-91]: We need to understand composition. Let f : { 0 , 1 } m → { 0 , 1 } , g : { 0 , 1 } n → { 0 , 1 } . The composition f ⋄ g : { 0 , 1 } m × n → { 0 , 1 } is n g f . m a ( f ⋄ g )( X ) . X .
The KRW conjecture g D ( f ) f f . a ( f ⋄ g )( X ) . X . . . . D ( g ) g g . . . . . . X 1 X m Clearly, D ( f ⋄ g ) ≤ D ( f ) + D ( g ) .
The KRW conjecture g D ( f ) f f . a ( f ⋄ g )( X ) . X . . . . D ( g ) g g . . . . . . X 1 X m Clearly, D ( f ⋄ g ) ≤ D ( f ) + D ( g ) . KRW conjecture: ∀ f, g : D ( f ⋄ g ) ≈ D ( f ) + D ( g ) .
The KRW conjecture g D ( f ) f f . a ( f ⋄ g )( X ) . X . . . . D ( g ) g g . . . . . . X 1 X m Clearly, D ( f ⋄ g ) ≤ D ( f ) + D ( g ) . KRW conjecture: ∀ f, g : D ( f ⋄ g ) ≈ D ( f ) + D ( g ) . Theorem [KRW91]: the conjecture implies that P � = NC 1 .
Outline Introduction 1 Known results 2 Proof strategy 3 Future directions 4
Karchmer-Wigderson relations Relate D ( f ) to complexity of a communication problem KW f .
Karchmer-Wigderson relations Relate D ( f ) to complexity of a communication problem KW f . The KW relation KW f is defined as follows: Alice gets x ∈ f − 1 (0) . Bob gets y ∈ f − 1 (1) . Clearly, x � = y , so ∃ i s.t. x i � = y i . Want to find such i .
Karchmer-Wigderson relations Relate D ( f ) to complexity of a communication problem KW f . The KW relation KW f is defined as follows: Alice gets x ∈ f − 1 (0) . Bob gets y ∈ f − 1 (1) . Clearly, x � = y , so ∃ i s.t. x i � = y i . Want to find such i . Theorem [KW88]: D ( f ) = C ( KW f ) .
Karchmer-Wigderson relations Relate D ( f ) to complexity of a communication problem KW f . The KW relation KW f is defined as follows: Alice gets x ∈ f − 1 (0) . Bob gets y ∈ f − 1 (1) . Clearly, x � = y , so ∃ i s.t. x i � = y i . Want to find such i . Theorem [KW88]: D ( f ) = C ( KW f ) . Only deterministic protocols!
Karchmer-Wigderson relations Relate D ( f ) to complexity of a communication problem KW f . The KW relation KW f is defined as follows: Alice gets x ∈ f − 1 (0) . Bob gets y ∈ f − 1 (1) . Clearly, x � = y , so ∃ i s.t. x i � = y i . Want to find such i . Theorem [KW88]: D ( f ) = C ( KW f ) . Only deterministic protocols! KRW conjecture: C ( KW f ⋄ g ) ≈ C ( KW f ) + C ( KW g )
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . .
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . .
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . .
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . .
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . .
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . .
KRW and KW Can we use KW games to attack the KRW conjecture? What does KW f ⋄ g look like? Recall: f ⋄ g maps { 0 , 1 } m × n to { 0 , 1 } . Alice Bob g g f f . . a . 0 1 . X b Y . . Hence, C ( KW f ⋄ g ) ≤ C ( KW f ) + C ( KW g ) . KRW conjecture: the obvious protocol is essentially optimal.
The universal relation The KRW conjecture is hard. [KRW91] suggested a starting point.
The universal relation The KRW conjecture is hard. [KRW91] suggested a starting point. The universal relation U n is: Alice gets x ∈ { 0 , 1 } n . Bob gets y ∈ { 0 , 1 } n . x � = y . Wish to find i s.t. x i � = y i .
The universal relation The KRW conjecture is hard. [KRW91] suggested a starting point. The universal relation U n is: Alice gets x ∈ { 0 , 1 } n . Bob gets y ∈ { 0 , 1 } n . x � = y . Wish to find i s.t. x i � = y i . Easy to prove: C (U n ) ≥ n .
The universal relation The KRW conjecture is hard. [KRW91] suggested a starting point. The universal relation U n is: Alice gets x ∈ { 0 , 1 } n . Bob gets y ∈ { 0 , 1 } n . x � = y . Wish to find i s.t. x i � = y i . Easy to prove: C (U n ) ≥ n . [KRW91] suggested to study U m ⋄ U n .
The composition of the universal relation [KRW91] suggested to study the composition U m ⋄ U n . Alice Bob a X b Y a � = b . If a i � = b i then X i � = Y i .
The composition of the universal relation [KRW91] suggested to study the composition U m ⋄ U n . Alice Bob a X b Y a � = b . If a i � = b i then X i � = Y i .
The composition of the universal relation [KRW91] suggested to study the composition U m ⋄ U n . Alice Bob a X b Y a � = b . If a i � = b i then X i � = Y i .
The composition of the universal relation [KRW91] suggested to study the composition U m ⋄ U n . Alice Bob a X b Y a � = b . If a i � = b i then X i � = Y i .
The composition of the universal relation Goal: C (U m ⋄ U n ) = C (U m ) + C (U n ) ≥ m + n . Alice Bob a X b Y
The composition of the universal relation Goal: C (U m ⋄ U n ) = C (U m ) + C (U n ) ≥ m + n . Challenge was met by [Edmonds-Impagliazzo-Rudich-S’gall-91]. Alice Bob a X b Y
The composition of the universal relation Goal: C (U m ⋄ U n ) = C (U m ) + C (U n ) ≥ m + n . Challenge was met by [Edmonds-Impagliazzo-Rudich-S’gall-91]. Alternative proof obtained by [H˚ astad-Wigderson-93]. Alice Bob a X b Y
Composing a function and the universal relation An analog of KRW conjecture for KW f ⋄ U n for any f . [Gavinsky-M-Weinstein-Wigderson-14] Alice Bob f f a 0 1 X b Y If a i � = b i then X j � = Y j . The obvious protocol works.
Composing a function and the universal relation An analog of KRW conjecture for KW f ⋄ U n for any f . [Gavinsky-M-Weinstein-Wigderson-14] Quantative improvement by [Koroth-M-18]. Alice Bob f f a 0 1 X b Y If a i � = b i then X j � = Y j . The obvious protocol works.
Composing any function and parity [Dinur-M-16]: (Re-)proved KRW conjecture for f ⋄ � n
Composing any function and parity [Dinur-M-16]: (Re-)proved KRW conjecture for f ⋄ � n Actually, this case was already implicit in [H˚ astad 98].
Composing any function and parity [Dinur-M-16]: (Re-)proved KRW conjecture for f ⋄ � n Actually, this case was already implicit in [H˚ astad 98]. However, our proof was very different, and more in line with the other works on the KRW conjecture.
Outline Introduction 1 Known results 2 Proof strategy 3 Future directions 4
Why should the obvious protocol be optimal? Alice Bob g g f f . . a . 0 1 . X b Y . .
Why should the obvious protocol be optimal? Alice Bob g g f f . . a . 0 1 . X b Y . . The players must solve KW g on some row.
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