Intro

Administrivia.

• Signup sheet.
• prerequisites: 6.046, 6.041/2, ability to do proofs
• dependence on 6.854
• narrower, deeper, less generally useful
• homework weekly (first friday)
• collaboration
• independent problems
• grading requirement
• term project
• books.

Randomized algorithms: make random choices during run. Main benefits:

• speed: may be faster than any deterministic
• even if not faster, often simpler (quicksort)
• sometimes, randomized is best
• sometime, randomized idea leads to deterministic algorithm

Distinguish average-cast analysis

• Probabilistic analysis assuming random input
• randomized algorithms do not assume random inputs
• so analyses are more applicable

We don't really use random numbers. But randomized algorithms break patterns we don't know are there.

• deterministic algorithm: works well except a few specific cases.
• But those are the ones you will encounter (Murphy)!
• randomized: almost always works well on any case
• but sometimes does bad on any case, so risky for life-threatening errors.

Course objective:

• Randomization is a general technique. Applies to all areas of CS.
• Underlying it is a common set of tools.
• Goal is to give familiarity with those tools so you can apply them to your own problems.
• To present tools, we draw appliations from many areas of CS: data structures, geometric algos, graph algos, parallel and distributed, number theory.
• Because so many, only a brief taste of each.
• But sufficient to go on alone.

Basic methodologies.

• Avoiding adversarial inputs
  • sorted quicksort list
  • a kind of random reordering (geometry---BSP)
  • hashing to same buckets
  • online algorithms
  • note: “adversarial” may mean “well structured” i.e. natural
• fingerprinting/verification
  • generate short random fingerprints for things
  • faster than comparing things
  • almost every fingerprint works
  • so a random one works
• random sampling. graph algs, computational geometry, median
  • fast way to find “typical” members
  • solve representative subproblem fast
  • extrapolate to solution of original problem
• load balancing
  • randomization spreads things out uniformly
  • parallel algs, routing, hashing
• symmetry breaking
  • random decisions keep everyone from doing the same thing
  • ethernet
  • deadlock avoidance in distributed systems (MUST randomize)
• Probabilistic existence proofs
  • thought experiment
  • prove an object is built with positive probability
  • guarantees object exists
  • makes search for algo worthwhile.

Today: 2 really basic principles:

• linearity of expectation
• product of event probabilities (independence)

Then some fundamental ideas:

• Kinds of randomized algorithms
• a bit of complexity

Quicksort

Items $S_1,\ldots,S_n$ to be sorted

• suppose could pick middle element: \[ T(n) = 2T(n/2)+O(n) = O(n\log n) \] works since divides into much smaller subproblems
• picking middle is hard. But an almost middle element is OK.
• pick random element. “probably” near middle and divides problem in two
• bound expected number of comparisons $C$
• $X_{ij}=1$ if compare $i$ to $j$
• linearity of expectation: $E[C]=\sum E[X_{ij}]$
• $E[X_{ij}] = p_{ij}$
• Consider smallest recursive call involving both $i$ and $j$.
• pivot must be one of $S_i,\ldots,S_j$. all equally likely
• $S_i$ and $S_j$ get compared if pivot is $S_i$ or $S_j$
• probability is at most $2/(j-i+1)$ (may have outer elements)
• analysis: $$ \begin{align*} \sum_{i=1}^n \sum_{j>i} p_{ij} &\le \sum_{i=1}^n \sum_{j>i} 2/(j-i+1)\\ &= \sum_{i=1}^n \sum_{k=1}^{n-i+1} 2/k\\ &\le 2\sum_{i=1}^n \sum_{k=1}^{n} 1/k\\ &\le 2nH_n\\ &= O(n\log n). \end{align*} $$
• (Define $H_n$, claim $O(\log n)$.)
• analysis holds for every input, doesn't assume random input
• we proved expected. can show high probability
• how did we pick a random elements? Depends on model.
• algorithm always works, but might be slow.

BSP

• linearity of expectation. hat check problem
• Rendering an image
  • render a collection of polygons (lines)
  • painters algorithm: draw from back to front; let front overwrite
  • need to figure out order with respect to user
• Sort?
  • Problem: items might be incomparable
  • Problem: might be cycles in sort order
  • Suggests need to split polygons
• define BSP.
  • BSP is a data structure that makes order determination easy
  • Build in preprocess step, then render fast.
  • Choose any hyperplane (root of tree), split lines onto correct side of hyperplane, recurse
  • If user is on side 1 of hyperplane, then nothing on side 2 blocks side 1, so paint it first. Recurse.
  • time=BSP size
  • User in DOOM, Quake, other FPS games
• sometimes must split to build BSP
• how limit splits?
• autopartitions
• random autopartitions
• analysis
  • $\indx(u,v)=k$ if $k$ lines block $v$ from $u$
  • $u \dashv v$ if $v$ cut by $u$ auto
  • probability $1/(1+\indx(u,v))$.
  • tree size is (by linearity of $E$) $$ \begin{eqnarray*} n+\sum 1/\indx(u,v) &\le & \sum_u 2H_n \end{eqnarray*} $$
• result: exists size $O(n\log n)$ auto
• gives randomized construction, time $O(n^2\log n)$
• best algorithm today: $O(n\log n)$ time to construct, size $O(n\log n)$.
• equally important, gives probabilistic existence proof of a small BSP
• so might hope to find deterministically.
• lower bound: $\Omega((\log n)/(\log\log n))$ so pretty tight.
• In 3D:
  • exist $n$ non-intersecting triangles with every autopartition $\Omega(n^2)$
  • Our same algorith, generalized the obvious way, uses $O(n^2)$ in expectation.

MinCut

• the problem
• deterministic algorithms
  • Max-flow
  • Gabow
• contraction
• conditionally independent events
• give/analyze
• repetition for better success probability (independent events)

Consequence: counting number of min-cuts.

Consequence: enumerating all min-cuts.

Min-cut implementation

• data structure for contractions
• alternative view---permutations.
• deterministic leaf algo
• finds all min-cuts whp

Monte Carlo vs. Las Vegas

• turn LV to MC by truncating
• turn MC to LV by certifying.
• if can't certify, dangerous!
• quicksort is LV
• mincut is MC