position paper on fairness. 1988
A rebuttal of Dijkstra's position on fairness.
the union of well-founded relations: An application of Koenig's
Lemma Feb 16, 1996
We are given two well-founded relations on the same set, and that their union is transitive. We show that the union is well-founded.
Sorting The Rows of a Matrix Preserves the Sorted Columns Feb 19, 1996
Given is a matrix whose columns are sorted. Show that if each row is sorted individually then the columns remain sorted.
Random Number Generation without Repetition Mar 27, 1996
We are given a function from naturals to naturals, which is applied repeatedly starting at a fixed seed. Floyd has shown how repetitions can be detected in this sequence. We give a proof of this result.
Counting Problem Communicated by Dijkstra April 9, 1996
Given that there are 25 boys and 25 girls. A party has 12 tables, each of which seats 2 boys and 2 girls. Thus, a party is attended by 24 boys and 24 girls. A boy sees 2 girls at his table in a party, and so do the girls. A set of parties, P, is feasible if each boy/girl sees different girls/boys in different parties in P. What is the size of the largest feasible set?
A puzzle on Termination April 9, 1996
Given is a 10 X 10 square grid. A cell is a square in the grid. Two cells are neighbors if they share a side. Initially 9 cells are chosen and colored red. Next, any cell may be colored red provided it has two red neighbors. Prove or disprove that the entire region can be colored red.
A puzzle on infinite sequences: An application of Koenig's Lemma April 12, 1996
Define a word to be any non-empty finite sequence of symbols. Each word is either good or bad. Given an infinite sequence of symbols, show that beyond some point, the sequence can be broken into words that are all good or that are all bad.
Satisfiability to Quadratic Programming May 15, 1996
We show that the boolean satisfiability problem can be reduced to the quadratic programming problem.
Useful Recurrence for Division June 20, 1996
We show a recurrence that is useful for division.
Grid Points, without Rabbits and Snakes Dec 18, 1996
For any finite set of grid points in the plane, we can colour each of the points either red or blue such that on each grid line the number of red points differs by at most 1 from the number of blue points.
are Infinitely Many Primes Mar 21, 1997
The following proof of the classical theorem is due to Dijkstra.
Multiple Copies under possibility of Failure April 4, 1997
It is required to copy a file N times; call each copy a clone of the original. The file consists of a sequence of symbols each of which is independently copied. Therefore, we consider the problem when the file consists of a single symbol.
Meeting Time Dec 5, 1997
We develop the appropriate conditions on the functions which are used in the common meeting time problem.
Dijkstra's Proof of Hall's Theorem Dec 22, 1997
This note contains my interpretation of a proof by Dijkstra of Hall's theorem.
Remarks on Hall's Theorem of Distinct Representatives Dec 30, 1997.
This is an observation by me on Hall's theorem.
Knowledge, Product and Sum
Jan 30, 1998
There are two parties, Product and Sum, who are given numbers p and s, respectively, where p = m x n and s = m+n, for some unknown integers m and n, each between 2 and 100. The parties deduce the values of m and n after a certain exchange. This note explains a solution due to Dijkstra (EWD-666).
A Proof about the Harmonic Series.
Aug 13, 1998
The following problem and its solution was shown to me by Carroll Morgan.
Mean is greater than or equal to Geometric Mean.
Sep 23, 1998
A short proof of this classical result.
Muddy Children Puzzle Oct 8, 1998
There is a finite group of children where each child is clean or dirty. No child knows if it is clean or dirty, but it can see if every other child is clean or dirty. It is common knowledge that there is at least one dirty child. In a round, (1) the children are asked: do you know if you are dirty, and (2) each of them responds with ``NO'', ``YES, I am dirty'', or ``YES, I am clean''. Responses are heard by all children. Rounds are repeated ad infinitum starting at round 0. Prove that a child who sees n dirty children, n \ge 0, will answer YES in round n, but no earlier.
Shortest Path as a Simulation Problem. Nov 13, 1998
Dijkstra's Shortest path algorithm can be seen as a simulation of a physical system.
for Division by 3 and 11. Dec 2, 1998
Show that for any natural number n, n mod 3 = r.n mod 3, where r.n is the sum of decimal digits in n. The rule for division by 11 is to start from the lowest digit of the number, and add and subtract the digits alternately.
Spanning Tree. Dec 12, 1998
This note develops two well-known algorithms for finding the minimum spanning tree.
property of Identity Function: An Exercise in Induction.
Sep 16, 1999
Let f be a function from naturals to naturals. It is given that for all n, f2(n) < f(n+1). Prove that f is the identity function. We will actually prove a generalization of this result.
Paradox, Cantor Diagonalization. Sep 28, 1999
Formal Proofs are simpler than their informal counterparts, for Russell Paradox and Cantor's Diagonalization argument.
Generated by a Stack Machine. Sep 28, 1999
An output string is computed as follows from an input string using an infinite stack. In each step either the next input symbol is added to the stack, or the top of the stack is moved to the output. We characterize the outputs for a given input.
proof by Erdos. Oct 1, 1999
A sequence of numbers whose length exceeds n2 contains either an ascending or a descending subsequence longer than n.
Prime Factorization Theorem. Feb 4, 2006
For every positive integer there is a unique bag of primes whose product equals that integer. The fact that there is a bag of primes corresponding to every positive integer is readily proven using induction. We prove the uniqueness part: distinct bags of primes have distinct products.
the Center of a Set of Points on a Curve. June 15, 2000
Given are a finite number of points on a simple closed curve; call these points anchors. It is required to find a point, called the center, so that the sum of distances between the center and the anchors is the minimum over all points.
the Strings of a Regular Expression. Aug 29, 2000
We develop an algorithm to enumerate the strings of a regular expression in increasing order.
A proof of quiescence of a distributed algorithm
Aug 30, 2000
An undirected connected finite graph has a natural number initially associated with each node. There is a distinguished node, anchor, whose value is always 0. A non-anchor node can make a move only if its value differs from v, which is 1 + the minimum value over all its neighbors; in that case it sets its value to v. A computation is an alternating sequence of states and moves, starting with the initial state. Show that each computation is finite.
Spans. Jan 22, 2001
Given is a sequence of integers. For any element, e, of the sequence, define its "span" to be the length of the longest segment ending at e where each element of the segment is at most e in value. This note contains development of a linear algorithm for computing all the spans.
Problem due to J Moore. Mar 28, 2001
The following problem was posed by J Moore during the faculty lunch today. Let there be two machines with two registers each, which can read/write a shared counter. Initially the counter holds the value 1 and all registers are empty. There are two atomic actions:
Parities of Binomial Coefficients
May 16, 2001
We give a formula for the parity of any binomial coefficient.
Isomorphism: An Exercise in Functional Programming
Sep 10, 2001
The problem is to decide if two unordered trees are the same.
A Note on EWD 1312
Sep 14, 2001
Integers h and k are disjoint provided in their binary representations no position has 1s in both h and k. (In comparing two integers, append leading zeroes to the binary representation of the smaller number to make their lengths identical.) Dijkstra proved that for positive h and k, among (h, k), (h, k-1), and (h-1, k), an even number of pairs are disjoint. In this note we prove this result using some elementary algebraic properties of disjointness.
A Note on EWD 967
November 9, 2003
Let S be a finite set which is closed with respect to a binary commutative and associative operation *, and that for all x and y, x*x*y=y. Show that the size of S is a power of 2.
Consensus Protocol in a Prison. February 2, 2004
A set of prisoners --assume there are at least 2-- are asked to play the following game by the warden. There is a room in the prison which has two switches; initially, the switches are in arbitrary positions. The warden will bring one prisoner at a time to the room, and the prisoner must flip one of the switches. The prisoners do not know the order in which they will be taken to the room, but they know that every prisoner will visit the room over and over until the end of the game. The game ends when some prisoner announces, ``every prisoner has been in this room at least once''. If the announcement is correct, all prisoners go free; if incorrect, they all are executed. The game continues until the announcement is made.
The prisoners are allowed to confer and decide on a protocol prior to the start of the game. Once the game starts, they are not allowed to communicate, nor can they find out who is being taken to the room. The problem is to devise a protocol for the prisoners.
Pairing Integers so that their sums are primes. January 31, 2005
For every even positive integer n, pair the integers up to n so that the sum of each pair is prime.
Some Facts about String Interleaving February 17, 2005
We prove commutativity, associativity and a distributivity property of string interleaving.
A puzzle from Peter Winkler July 19, 2006
A secret is a triple where each component is a natural number below n, for some given n. A guess is a triple of the same form. A guess has an outcome which is revealed to the guesser: it succeeds if it matches at least two corresponding components of the secret, and fails otherwise. What is the minimum number of guesses required to succeed for n=8?
Chameleons Sept. 8, 2009
There are 3 piles of chips. A step consists of removing one chip each from two different piles and adding both chips to the third pile. The game terminates (reaches a final state) when no more steps can be taken, i.e., there are at least two empty piles. It is required to devise a strategy to arrive at a terminating state, or prove that no such strategy exists for the given initial state. This puzzle first appeared in the Comm. of ACM in the form of Chameleons of 3 different colors. I have made it less colorful by converting it to piles of chips.
Monty Hall game Aug. 10, 2011
Four short proofs for the famous Monty Hall game.
Boolean Equalities and Inequalities. Nov. 1, 2011
Given a set of boolean variables, and a set of equalities and inequalities among those variables (called ``facts''), it is required to determine the relationship, if any, among pairs of variables (called ``queries''). We consider two versions of this problems, one when the queries are given after all the facts have been presented (off-line) and when the queries and facts are intermingled (on-line). For the off-line version we give a linear algorithm, and for the on-line version a quasi-linear one by modifying the well known union-find algorithm.
Coordinates to Events: Solving Combinatorial problems using Discrete
Event Simulation. (with David Kitchin and John
Thywissen) Nov. 1, 2011
This paper is inspired by the "event list" mechanism in discrete event simulations. We argue that descriptions of many combinatorial algorithms can be simplified by casting the solution in terms of processing events according to some order. We propose generalizations of the event list mechanism, and show their applications in problems from graph theory and computational geometry.
A Proof of the Infinite Ramsey Theorem July 31, 2012
Lights with their Switches Sep. 07, 2012
Given is a set of switches and an equal number of lights where each switch controls exactly one light and each light is controlled by exactly one switch. The wiring diagram is unavailable and the wiring itself is hidden. A step consists of selecting some number of switches and turning them on, and, presumably, noting the lights that come on as a result. It is required to determine which switch controls which light using a minimum number of steps.
Digital Cash Aug. 24, 2012
We propose a scheme for management of digital cash that mimics the current physical management. In particular, any one can verify the authenticity of a digital bill, no one can manufacture or double-spend a bill, a transaction between two parties does not reveal the identities of the parties to others, and the bills are maintained by a distributed set of (trusted) servers that may belong to many financial institutions, much like the banks of today. Additionally, failure of any one client or server does not affect the rest of the system.
Guessing a Hat color out of N colors Sep. 14, 2012
The following problem was communicated to me by Mike Starbird. There are N persons each of whom has a hat on his head. There are N possible hat colors. Not every color appears on someone's head. Every one can see the colors of all others' hats, but not his own. Each person guesses the color of his hat (writing it down on a piece if paper, say). Devise a protocol so that at least one person guesses his hat color correctly.
A Proof of the Boyer-Moore Majority Protocol June 11,
This note is inspired by discussions with Greg Plaxton who has observed that the Boyer-Moore Majority Protocol provides an excellent example for teaching program verification in an undergraduate course. I write out the proof in detail here. The first program uses certain abstract data structures. Its proof is relatively easy to construct. The next program is a refinement, replacing the abstract data structures by concrete representations.