Lecture 21

Probabilistic Turing Machines and Bounded-error Probabilistic Polynomial Time

Definition A probabilistic Turing machine is a non-deterministic Turing machine with \(2\) transition functions \(\delta_0\) and \(\delta_1\). At each step of the computation, the Turing machine flips a coin to determine whether if follows the transition from \(\delta_0\) or \(\delta_1\).

Definition The running time of a probabilistic Turing machine \(M\) is a function \(t:\mathbb{N}\rightarrow\mathbb{N}\) such that for every input \(x\in\{0,1\}^n\) and for each possible outcome of the coin flips, \(M\) terminates after \(t(n)\) steps.

Definition The probabilistic Turing machine \(M\) decides \(L\subseteq\{0,1\}^*\) with bounded error if it always halts and

  1. when \(x\in L\), \(M\) accepts with probability \(\geq \frac{2}{3}\).
  2. when \(x\not\in L\), \(M\) rejects with probability \(\geq \frac{2}{3}\).

Definition The language \(L\subseteq\{0,1\}^*\) is in BPP if it is decided with bounded-error by a polynomial-time probabilistic Turing machine.

Thesis BPP captures all languages that can be computed efficiently in any reasonable model of randomized computations.

Alternative Randomness Models

Definition A super general probabilistic Turing machine is a non-deterministic Turing machine where each transition is associated with a number \(0< p <1\) that determines the probability that the Turing machine follows this transition at appropriate steps.

Proposition If \(L\) can be decided by a super general probabilistic Turing machine with running time \(t\) and all the transition probabilities of \(M\) are of the form \(p=\frac{c}{2^k}\) \(c,k\in\mathbb{N}\), then it can be decided by a probabilistic Turing machine \(M'\) with running time \(t'=O(t(n))\).

Proof Build a binary tree of depth \(max(k)\) for each step.

Theorem If \(L\) can be decided by a polynomial-time super general probabilistic Turing machine whose transition probabilities are polynomial-time computable, then \(L\in BPP\).

Error Reduction

Proposition If \(M\) decides \(L\) with bounded error \(\frac{1}{3}\) then there exists a probabilistic Turning machine \(M'\) that decides \(L\) with error at most \(\frac{7}{27}\)

Proof Have \(M'\) run \(e\) independent copies of \(M\) and take the majority. The probability that at least two executions of \(M\) are correct is \((\frac{2}{3})^3+3(\frac{1}{3})(\frac{2}{3})^2=\frac{20}{27}\)

Theorem If \(M\) decides \(L\) with bounded error \(\frac{1}{3}\) and running time \(t\), then for any \(\epsilon >0\) there is a probabilistic Turing machine \(M'\) with bounded error \(\epsilon\) and running time \(t'=O(t(n))\)

Proof Again by majority of enough copies of \(M\).

Definition \(L\) is in \(RP\) if there is a polynomial-time probabilistic Turing machine such that

  1. when \(x\in L\), then \(M\) accepts with probability \(\geq\frac{1}{2}\)
  2. when \(x\not\in L\), then \(M\) always rejects.

Definition \(L\) is in \(PerfectRP\) if there is a polynomial-time probabilistic Turing machine \(M\) such that \(M\) accepts iff \(x\in L\).

Proposition \(PerfectRP=P\)

Average-case Running Time

Definition The average-case running time of a probabilistic Turing machine is a function \(t:\mathbb{N}\rightarrow\mathbb{N}\) such that for every input \(x\in\{0,1\}^n\), the expected running time of \(M\) on \(x\) is at most \(t(n)\).

Definition \(L\in ZPP\) (Zero-error Probabilistic Polynomial Time) if there is a probabilistic Turing machine \(M\) with polynomial average-case running time such that \(M\) accepts \(x\) if \(x\in L\).

Theorem If \(L\) is decidable with bounded error by a probabilistic Turing machine with polynomial average-case running time, then \(L\in BPP\)

Proof Truncate (terminate) long-running Turing machine and guess.