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Informally, an algorithm is a set of explicit instructions used to solve a problem (e.g. Euclid's algorithm for computing the greatest common divisor of two integers). For more specific questions on algorithms, this tag may be used in conjunction with the approximation-algorithms, algorithmic-randomness and algorithmic-topology tags.

3 votes
1 answer
402 views

How "often" does LLL-reduction produce "optimal" result ? Is there condition (or informal un...

Consider some lattice in R^4 (C^4) or C^8. Famous "lattice reduction" procedures (like LLL latice reduction) produces some "reduced basis". However in general there results are not "the best reduced …
Alexander Chervov's user avatar
4 votes
2 answers
1k views

diameter of Voronoi cell of the lattice ? What about R^n ? What about small n =2,3,4 ?What a...

Consider a lattice in R^n. Consider Voronoi cell of it. What is known about diameter ? About the shape ? What are good references ? As far as I understand they are not easy to compute. May be in s …
Alexander Chervov's user avatar
1 vote
0 answers
198 views

How to find closest point to restricted lattice on the plane ? ( m*h1 + n*h2, for 0<m,n<N)

Consider finite piece of lattice i.e. points of the form m*h1 + n*h2, for 0<m,n<N h1, h2 -some vectors. Consider some point "P" on the plane. How to find (m,n) such that m*h1 + n*h2 is closest to "P" …
Alexander Chervov's user avatar
2 votes
2 answers
206 views

how to find vertex of parallelotope closest to given point P in R^n ? (Or minimize quadrati...

What algorithms (except of brute force) can be suggested to find the closest vertex of paralleloptope to "P" ? Is it NP ? …
Alexander Chervov's user avatar
5 votes
2 answers
965 views

Lattice reduction in R^3 (R^4) or what is fundamental domain for SL(3,Z) , (SL(4,Z)) ?

What are the algorithms to find such a lattice reductions ? …
Alexander Chervov's user avatar
6 votes
1 answer
2k views

Closest vector problem (=nearest lattice point) is trivial for "reduced lattice" ?

Consider some lattice in R^n. Take some point "P" in R^n (which does not belong to this lattice in general). The problem is to find "nearest" lattice point. The problem is known NP-hard in general it …
Alexander Chervov's user avatar
21 votes
5 answers
3k views

How can you find an integer coefficient polynomial knowing its values only at a few points (...

Example: How can you guess a polynomial $p$ if you know that $p(2) = 11$? It is simple: just write 11 in binary format: 1011 and it gives the coefficients: $p(x) = x^3+x+1$. Well, of course, this po …
Alexander Chervov's user avatar
2 votes
5 answers
3k views

Algorithm to solve Sokoban-like game on graphs - move chips from one set of vertices to another

The question is close to the Sokoban game (thanks to Dima Pasechnik !), but a little different in details. Consider a directed graph (multi-graph). Consider some set of marked chips (chip1, chipe2,.. …
Alexander Chervov's user avatar
3 votes
2 answers
571 views

Complexity of establishing finite groups (non)-isomorphism ?

Question Given two finite groups G and H of the same order N what are the algorithms and what is their complexity (in terms of N) to check is G isomorphic to H or not ? …
Alexander Chervov's user avatar
15 votes
4 answers
4k views

Algorithm to check is representation irreducible ? Algorithm to decompose the reducible one ?

Question 2 Given a representation of a finite group, what algorithms can be used to decompose it to the direct sum of irreducibles) ? …
Alexander Chervov's user avatar
1 vote
0 answers
8 views

Algorithms (or packages) to find recurrence relations for given sequence of q-polynomials?

Assume we have sequence of polynomials : $P_i(q)$ - each term is polynomial in $q$. (With integer coefficients, but hopefully it is not important). We expect that there exists recurrence relation: $P_ …
Alexander Chervov's user avatar