93 research outputs found
On Termination of Integer Linear Loops
A fundamental problem in program verification concerns the termination of
simple linear loops of the form x := u ; while Bx >= b do {x := Ax + a} where x
is a vector of variables, u, a, and c are integer vectors, and A and B are
integer matrices. Assuming the matrix A is diagonalisable, we give a decision
procedure for the problem of whether, for all initial integer vectors u, such a
loop terminates. The correctness of our algorithm relies on sophisticated tools
from algebraic and analytic number theory, Diophantine geometry, and real
algebraic geometry. To the best of our knowledge, this is the first substantial
advance on a 10-year-old open problem of Tiwari (2004) and Braverman (2006).Comment: Accepted to SODA1
Error-Correction Coding and Decoding: Bounds, Codes, Decoders, Analysis and Applications
Coding; Communications; Engineering; Networks; Information Theory; Algorithm
Cyclotomic Identity Testing and Applications
We consider the cyclotomic identity testing problem: given a polynomial
, decide whether is
zero, for a primitive complex -th root of unity and
integers . We assume that and are
represented in binary and consider several versions of the problem, according
to the representation of . For the case that is given by an algebraic
circuit we give a randomized polynomial-time algorithm with two-sided errors,
showing that the problem lies in BPP. In case is given by a circuit of
polynomially bounded syntactic degree, we give a randomized algorithm with
two-sided errors that runs in poly-logarithmic parallel time, showing that the
problem lies in BPNC. In case is given by a depth-2 circuit
(or, equivalently, as a list of monomials), we show that the cyclotomic
identity testing problem lies in NC. Under the generalised Riemann hypothesis,
we are able to extend this approach to obtain a polynomial-time algorithm also
for a very simple subclass of depth-3 circuits. We complement
this last result by showing that for a more general class of depth-3
circuits, a polynomial-time algorithm for the cyclotomic
identity testing problem would yield a sub-exponential-time algorithm for
polynomial identity testing. Finally, we use cyclotomic identity testing to
give a new proof that equality of compressed strings, i.e., strings presented
using context-free grammars, can be decided in coRNC: randomized NC with
one-sided errors
A STUDY OF LINEAR ERROR CORRECTING CODES
Since Shannon's ground-breaking work in 1948, there have been two main development streams
of channel coding in approaching the limit of communication channels, namely classical coding
theory which aims at designing codes with large minimum Hamming distance and probabilistic
coding which places the emphasis on low complexity probabilistic decoding using long codes built
from simple constituent codes. This work presents some further investigations in these two channel
coding development streams.
Low-density parity-check (LDPC) codes form a class of capacity-approaching codes with sparse
parity-check matrix and low-complexity decoder Two novel methods of constructing algebraic binary
LDPC codes are presented. These methods are based on the theory of cyclotomic cosets, idempotents
and Mattson-Solomon polynomials, and are complementary to each other. The two methods
generate in addition to some new cyclic iteratively decodable codes, the well-known Euclidean and
projective geometry codes. Their extension to non binary fields is shown to be straightforward.
These algebraic cyclic LDPC codes, for short block lengths, converge considerably well under iterative
decoding. It is also shown that for some of these codes, maximum likelihood performance may
be achieved by a modified belief propagation decoder which uses a different subset of 7^ codewords
of the dual code for each iteration.
Following a property of the revolving-door combination generator, multi-threaded minimum
Hamming distance computation algorithms are developed. Using these algorithms, the previously
unknown, minimum Hamming distance of the quadratic residue code for prime 199 has been evaluated.
In addition, the highest minimum Hamming distance attainable by all binary cyclic codes
of odd lengths from 129 to 189 has been determined, and as many as 901 new binary linear codes
which have higher minimum Hamming distance than the previously considered best known linear
code have been found.
It is shown that by exploiting the structure of circulant matrices, the number of codewords
required, to compute the minimum Hamming distance and the number of codewords of a given
Hamming weight of binary double-circulant codes based on primes, may be reduced. A means
of independently verifying the exhaustively computed number of codewords of a given Hamming
weight of these double-circulant codes is developed and in coiyunction with this, it is proved that
some published results are incorrect and the correct weight spectra are presented. Moreover, it is
shown that it is possible to estimate the minimum Hamming distance of this family of prime-based
double-circulant codes.
It is shown that linear codes may be efficiently decoded using the incremental correlation Dorsch
algorithm. By extending this algorithm, a list decoder is derived and a novel, CRC-less error detection
mechanism that offers much better throughput and performance than the conventional ORG
scheme is described. Using the same method it is shown that the performance of conventional CRC
scheme may be considerably enhanced. Error detection is an integral part of an incremental redundancy
communications system and it is shown that sequences of good error correction codes,
suitable for use in incremental redundancy communications systems may be obtained using the
Constructions X and XX. Examples are given and their performances presented in comparison to
conventional CRC schemes
The Segal conjecture for topological Hochschild homology of complex cobordism
We study the C_p-equivariant Tate construction on the topological Hochschild
homology THH(B) of a symmetric ring spectrum B by relating it to a topological
version R_+(B) of the Singer construction, extended by a natural circle action.
This enables us to prove that the fixed and homotopy fixed point spectra of
THH(B) are p-adically equivalent for B = MU and BP. This generalizes the
classical C_p-equivariant Segal conjecture, which corresponds to the case B =
S.Comment: Accepted for publication by the Journal of Topolog
Complexity measures for classes of sequences and cryptographic apllications
Pseudo-random sequences are a crucial component of cryptography, particularly
in stream cipher design. In this thesis we will investigate several measures of
randomness for certain classes of finitely generated sequences.
We will present a heuristic algorithm for calculating the k-error linear complexity
of a general sequence, of either finite or infinite length, and results on the
closeness of the approximation generated.
We will present an linear time algorithm for determining the linear complexity
of a sequence whose characteristic polynomial is a power of an irreducible element,
again presenting variations for both finite and infinite sequences. This algorithm
allows the linear complexity of such sequences to be determined faster than was
previously possible.
Finally we investigate the stability of m-sequences, in terms of both k-error
linear complexity and k-error period. We show that such sequences are inherently
stable, but show that some are more stable than others
Path isomorphisms between quiver Hecke and diagrammatic Bott-Samelson endomorphism algebras
We construct an explicit isomorphism between (truncations of) quiver Hecke
algebras and Elias-Williamson's diagrammatic endomorphism algebras of
Bott-Samelson bimodules. As a corollary, we deduce that the decomposition
numbers of these algebras (including as examples the symmetric groups and
generalised blob algebras) are tautologically equal to the associated
-Kazhdan-Lusztig polynomials, provided that the characteristic is greater
than the Coxeter number. We hence give an elementary and more explicit proof of
the main theorem of Riche-Williamson's recent monograph and extend their
categorical equivalence to cyclotomic Hecke algebras, thus solving
Libedinsky-Plaza's categorical blob conjecture
Usability of structured lattices for a post-quantum cryptography: practical computations, and a study of some real Kummer extensions
Lattice-based cryptography is an excellent candidate for post-quantum cryptography, i.e. cryptosystems which are resistant to attacks run on quantum computers. For efficiency reason, most of the constructions explored nowadays are based on structured lattices, such as module lattices or ideal lattices. The security of most constructions can be related to the hardness of retrieving a short element in such lattices, and one does not know yet to what extent these additional structures weaken the cryptosystems. A related problem – which is an extension of a classical problem in computational number theory – called the Short Principal Ideal Problem (or SPIP), consists of finding a short generator of a principal ideal. Its assumed hardness has been used to build some cryptographic schemes. However it has been shown to be solvable in quantum polynomial time over cyclotomic fields, through an attack which uses the Log-unit lattice of the field considered. Later, practical results showed that multiquadratic fields were also weak to this strategy.
The main general question that we study in this thesis is To what extent can structured lattices be used to build a post-quantum cryptography
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