Week 8: Second Moment Method
Transcript of Week 8: Second Moment Method
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The Probabilistic Method
Joshua BrodyCS49/Math59
Fall 2015
Week 8: Second Moment Method
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Reading Quiz
(A) a set of graphs
(B) a set of graphs closed under addition of edges
(C) a set of graphs closed under addition of vertices
(D) a set of graphs closed under isomorphism
(E) None of the above
What is a graph property?
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Reading Quiz
(A) a set of graphs
(B) a set of graphs closed under addition of edges
(C) a set of graphs closed under addition of vertices
(D) a set of graphs closed under isomorphism
(E) None of the above
What is a graph property?
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The First Moment Method
(1) Define bad events BADi
(2) BAD := ∪i BADi
(3) bound Pr[BADi] ≤ 𝜹
(4) Compute # bad events ≤ m(5) union bound:
Pr[BAD] ≤ m𝜹 < 1
(6) ∴ Pr[GOOD] > 0
(1) Zi: indicator var for BADi
(2) Z := ∑i Zi
(3) E[Zi] = Pr[BADi] ≤ 𝜹
(4) Compute # bad events ≤ m
(5) E[Z] = E[Zi] ≤ m𝜹 < 1
(6) ∴ Z = 0 w/prob > 0
Basic Method: as First Moment Method:
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Exploiting Expected Value
Suppose X is non-negative, integer random variable
Fact: Pr[X > 0] ≤ E[X]
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Exploiting Expected Value
Suppose X is non-negative, integer random variable
Fact: Pr[X > 0] ≤ E[X]
•If E[X] < 1, then Pr[X=0] > 0•If E[X] = o(1), then Pr[X=0] = 1-o(1)•If E[X] →∞, then ???
Consequences:
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More on The Second Moment Method
Theorem: Pr[X = 0] ≤ Var[X]/E[X]2
•use Chebyshev’s Inequality with ! := E[X]proof:
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More on The Second Moment Method
Theorem: Pr[X = 0] ≤ Var[X]/E[X]2
• If Var[X] = o(E[X]2), then Pr[X=0] = o(1)Consequences:
•use Chebyshev’s Inequality with ! := E[X]proof:
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More on The Second Moment Method
Theorem: Pr[X = 0] ≤ Var[X]/E[X]2
• If Var[X] = o(E[X]2), then Pr[X=0] = o(1)Consequences:
•use Chebyshev’s Inequality with ! := E[X]proof:
X > 0 “almost always”
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More on The Second Moment Method
Theorem: Pr[X = 0] ≤ Var[X]/E[X]2
• If Var[X] = o(E[X]2), then Pr[X=0] = o(1)• If Var[X] = o(E[X]2), then X ~ E[X] almost always.
Consequences:
•use Chebyshev’s Inequality with ! := E[X]proof:
X > 0 “almost always”
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Random Graphs
G ~ G(n,p) : random graph on n vertices V = {1, ..., n} each edge (i,j) ∈ E independently with prob. p
[Erdős-Rényi 60]
G(n,p) : probability distributionG : random variable
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Clicker Question
(A) S, T share at least one vertex
(B) S, T share at least one edge
(C) S, T share at least two vertices
(D) (A) and (B)
(E) (B) and (C)
When are AS and AT not independent?
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Clicker Question
(A) S, T share at least one vertex
(B) S, T share at least one edge
(C) S, T share at least two vertices
(D) (A) and (B)
(E) (B) and (C)
When are AS and AT not independent?
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The Probabilistic Method