# All Questions

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### Max-cut with negative weight edges

Let $G = (V, E, w)$ be a graph with weight function $w:E\rightarrow \mathbb{R}$. The max-cut problem is to find: $$\arg\max_{S \subset V} \sum_{(u,v) \in E : u \in S, v \not \in S}w(u,v)$$ If the ...
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### Toy Examples for Plotkin-Shmoys-Tardos and Arora-Kale solvers

I would like to understand how the Arora-Kale SDP solver approximates the Goemans-Williamson relaxation in nearly linear time, how the Plotkin-Shmoys-Tardos solver approximates fractional "packing" ...
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### NP-Completeness of the decision problem for the generalized 15-puzzle

I am interested in the natural generalization of the famous 15-puzzle, where you have to slide blocks until you have sorted all given numbers (usally there is a gap of 1 block). Now the ...
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### Efficiently computable function as a counter-example to Sarnak's Mobius conjecture

Recently Gil Kalai and Dick Lipton both wrote a nice article on an interesting conjecture proposed by Peter Sarnak, an expert in number theory and Riemann Hypothesis. Conjecture. Let $\mu(k)$ be ...
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### Hardness jumps in computational complexity?

Minimum bandwidth problem is to a find an ordering of graph nodes on integer line that minimizes the largest distance between any two adjacent nodes. A $k$-caterpillar is a tree formed from main path ...
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### Alan Turing's Contributions to Computer Science

Alan Turing, one of the pioneers of (theoretical) computer science, made many seminal scientific contributions to our field, including defining Turing machines, the Church-Turing thesis, ...
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### Approximation algorithms for problems in P

One usually thinks about approximating solutions (with guarantees) to NP-hard problems. Is there any research going on in approximating problems already known to be in P? This might be a good idea for ...
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### Concepts in theoretical CS that would be approachable ages 8-14

Guessing it's unlikely a common question, but wondering if anyone has seen material that was clearly made to address this audience in a meaningful way.
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In my academic career, I've read quite a few academic papers on various computer science topics. Many of which involve an implementation and some assessment of that implementation, yet I have found ...
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### Comparison-based data structure for finding items

Is there a data structure that takes an unordered array of $n$ items, performs preprocessing in $O(n)$ and answers queries: is there some element $x$ on the list, each query in worst time $O(\log n)$? ...
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### Turing Machine restrictions that render halting decidable

If one restricts Turing Machines to a finite tape (i.e., to use bounded space $S$), then the halting problem is decidable, essentially because after a number of steps (which can be calculated from the ...
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### Consequences of Factoring being in P?

Factoring is not known to be NP-complete. This question asked for consequences of Factoring being NP-complete. Curiously, no one asked for consequences of Factoring being in P (maybe because such a ...
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### complexity of greatest common divisor (gcd)

Consider the following counting problem (or the associated decision problem): Given two positive integers encoded in binary, compute their greatest common divisor (gcd). What is the smallest ...
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### Type classes vs object interfaces

I don't think I understand type classes. I'd read somewhere that thinking of type classes as "interfaces" (from OO) that a type implements is wrong and misleading. The problem is, I'm having a problem ...
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### Why would one ever use an Octree over a KD-tree?

I have some experience in scientific computing, and have extensively used kd-trees for BSP (binary space partitioning) applications. I have recently become rather more familiar with octrees, a similar ...
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### Cohomological approach to boolean complexity

A few years ago, there was some work by Joel Friedman relating lower circuit bounds to Grothendieck cohomology (see papers: http://arxiv.org/abs/cs/0512008, http://arxiv.org/abs/cs/0604024). Has this ...
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### Status of Impagliazzo's Worlds?

In 1995, Russell Impagliazzo proposed five complexity worlds: 1- Algorithmica: $P=NP$ with all the amazing consequences. 2- Heuristica: $NP$-complete problems are hard in the worst-case ($P \ne NP$) ...
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### What is the Volume of Information?

This question was asked to Jeannette Wing after her PCAST presentation on computer science. “From a physics perspective, is there a maximum volume of information we can have?” (a nice challenge ...
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### NTIME(n^k) ≠ DTIME(n^k) ?

In "On determinism versus nondeterminism and related problems" (Proc. IEEE FOCS, pages 429–438, 1983), Paul, Pippenger, Szemerédi and Trotter proved that $\mathsf{NTIME}(n)\neq\mathsf{DTIME}(n)$. ...
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### “Steve's class”: origin of SC

We "know" that $\mathsf{SC}$ is named for Steve Cook and $\mathsf{NC}$ is named for Nick Pippenger. If I'm not mistaken, Steve Cook named NC in honor of Nick Pippenger, and I was told that the reverse ...
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### $BQP$ vs $QMA$?

The central problem of complexity theory is arguably $P$ vs $NP$. However, since Nature is quantum, it would seem more natural to consider the classes $BQP$ (ie decision problems solvable by a ...
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### Book on Probability

While I have passed some courses on probability theory, both in the high school and the university, I have a hard time reading TCS papers when it comes to probability. It seems that the authors of ...
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### What is the quantum computational model?

I have occasionally heard people talk about quantum algorithms and about states and the ability to consider multiple possibilities at once, but I have never managed to get someone to explain the ...
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### What is the smallest Turing machine where it is unknown if it halts or not?

I know that the halting problem is undecidable in general but there are some Turing machines that obviously halt and some that obviously don't. Out of all possible turing machines what is the smallest ...
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### Books on automata theory for self-study

I need a finite automata theory book with lots of examples that I can use for self-study and to prepare for exams.
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### Programming languages for efficient computation

It is impossible to write a programming language that allows all machines that halt on all inputs and no others. However, it seems to be easy to define such a programming language for any standard ...
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### Evidence that PPAD is hard?

There is often-quoted philosophical justification for believing that P != NP even without proof. Other complexity classes have evidence that they are distinct, because if not, there would be "...
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### Algorithmic lens in the social sciences

Looking at questions through the algorithmic lens (i.e. from an algorithmic or complexity point of view) has become useful in disciplines outside of the 'standard domain' of computer science. In ...
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### Is there a stable heap?

Is there a priority queue data structure that supports the following operations? Insert(x, p): Add a new record x with priority p StableExtractMin(): Return and delete the record with minimum ...
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### Research and open challenges in Programming Language Theory

In the spirit of some general discussions like this one, I'm opening this thread with the intention to gather opinions on what are the open challenges and hot topics in research on programming ...
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### Hardness of approximation assuming NP != coNP

Two of the common assumptions for proving hardness of approximation results are $P \neq NP$ and Unique Games Conjecture. Are there any hardness of approximation results assuming $NP \neq coNP$ ? I am ...
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### An Anthology of Complexity Assumptions

In the paper The Random Oracle Hypothesis Is False, the authors (Chang, Chor, Goldreich, Hartmanis, Håstad, Ranjan, and Rohatgi) discuss the implications of the random-oracle hypothesis. They argue ...
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### How did TCS become conference-oriented rather than journal-oriented?

Disclaimer: I can only vouch for my research fields, namely formal methods, semantics and programming language theory. The situation is possibly different in other parts of the discipline. It seems ...
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### Does LOGLOG = NLOGLOG?

Define LOGLOG as the class of languages which can be computed in space O(loglog n) by a deterministic Turing machine (with two-way access to the input). Similarly define NLOGLOG as the class of ...
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### Is Gap-3SAT NP-complete even for 3CNF formulas where no pair of variables appears in significantly more clauses than the average?

In this question, a 3CNF formula means a CNF formula where each clause involves exactly three distinct variables. For a constant 0<s<1, Gap-3SATs is the following promise problem: Gap-3SATs ...