Computer Algebra System Magma Research Articles

Minimization of hypersurfaces

Let F ∈ Z [ x 0 , … , x n ] F \in \mathbb {Z}[x_0, \ldots , x_n] be homogeneous of degree d d and assume that F F is not a ‘nullform’, i.e., there is an invariant I I of forms of degree d d in n + 1 n+1 variables such that I ( F ) ≠ 0 I(F) \neq 0 . Equivalently, F F is semistable in the sense of Geometric Invariant Theory. Minimizing F F at a prime p p means to produce T ∈ M a t ( n + 1 , Z ) ∩ G L ( n + 1 , Q ) T \in Mat(n+1, \mathbb {Z}) \cap GL(n+1, \mathbb {Q}) and e ∈ Z ≥ 0 e \in \mathbb {Z}_{\ge 0} such that F 1 = p − e F ( [ x 0 , … , x n ] ⋅ T ) F_1 = p^{-e} F([x_0, \ldots , x_n] \cdot T) has integral coefficients and v p ( I ( F 1 ) ) v_p(I(F_1)) is minimal among all such F 1 F_1 . Following Kollár [Electron. Res. Announc. Amer. Math. Soc. 3 (1997), pp. 17–27], the minimization process can be described in terms of applying weight vectors w ∈ Z ≥ 0 n + 1 w \in \mathbb {Z}_{\ge 0}^{n+1} to F F . We show that for any dimension n n and degree d d , there is a complete set of weight vectors consisting of [ 0 , w 1 , w 2 , … , w n ] [0,w_1,w_2,\dots ,w_n] with 0 ≤ w 1 ≤ w 2 ≤ ⋯ ≤ w n ≤ 2 n d n − 1 0 \le w_1 \le w_2 \le \dots \le w_n \le 2 n d^{n-1} . When n = 2 n = 2 , we improve the bound to d d . This answers a question raised by Kollár. These results are valid in a more general context, replacing Z \mathbb {Z} and p p by a PID R R and a prime element of R R . Based on this result and a further study of the minimization process in the planar case n = 2 n = 2 , we devise an efficient minimization algorithm for ternary forms (equivalently, plane curves) of arbitrary degree d d . We also describe a similar algorithm that allows to minimize (and reduce) cubic surfaces. These algorithms are available in the computer algebra system Magma.

Representation of non-special curves of genus 5 as plane sextic curves and its application to finding curves with many rational points

In algebraic geometry, it is important to provide effective parametrizations for families of curves, both in theory and in practice. In this paper, we present such an effective parametrization for the moduli of genus-5 curves that are neither hyperelliptic nor trigonal. Subsequently, we construct an algorithm for a complete enumeration of non-special genus-5 curves having more rational points than a specified bound, where “non-special curve” means that the curve is non-hyperelliptic and non-trigonal with mild singularities of the associated sextic model that we propose. As a practical application, we implement this algorithm using the computer algebra system MAGMA, specifically for curves over the prime field of characteristic 3.

Tensor isomorphism by conjugacy of Lie algebras

We introduce an algorithm to decide isomorphism between tensors. The algorithm uses the Lie algebra of derivations of a tensor to compress the space in which the search takes place to a so-called densor space. To make the method practicable we give a polynomial-time algorithm to solve a generalization of module isomorphism for a common class of Lie modules. As a consequence, we show that isomorphism testing is in polynomial time for tensors whose derivation algebras are classical Lie algebras and whose densor spaces are 1-dimensional. The method has been implemented in the Magma computer algebra system.

Polarized Rigid Del Pezzo Surfaces in Low Codimension

We provide explicit graded constructions of orbifold del Pezzo surfaces with rigid orbifold points of type ki×1ri(1,ai):3≤ri≤10,ki∈Z≥0 as well-formed and quasismooth varieties embedded in some weighted projective space. In particular, we present a collection of 147 such surfaces such that their image under their anti-canonical embeddings can be described by using one of the following sets of equations: a single equation, two linearly independent equations, five maximal Pfaffians of 5×5 skew symmetric matrix, and nine 2×2 minors of size 3 square matrix. This is a complete classification of such surfaces under certain carefully chosen bounds on the weights of ambient weighted projective spaces and it is largely based on detailed computer-assisted searches by using the computer algebra system MAGMA.

Exact p-adic computation in Magma

We describe a new arithmetic system for the Magma computer algebra system for working with p-adic numbers exactly, in the sense that numbers are represented lazily to infinite p-adic precision. This is the first highly featured such implementation. This has the benefits of increasing user-friendliness and speeding up some computations, as well as forcibly producing provable results. We give theoretical and practical justification for its design and describe some use cases. The intention is that this article will be of benefit to anyone wanting to implement similar functionality in other languages.

Computational Approach to Enumerate Non-hyperelliptic Superspecial Curves of Genus 4

In this paper we enumerate nonhyperelliptic superspecial curves of genus $4$ over prime fields of characteristic $p\le 11$. Our algorithm works for nonhyperelliptic curves over an arbitrary finite field in characteristic $p \ge 5$. We execute the algorithm for prime fields of $p\le 11$ with our implementation on a computer algebra system Magma. Thanks to the fact that the cardinality of $\mathbb{F}_{p^a}$-isomorphism classes of superspecial curves over $\mathbb{F}_{p^a}$ of a fixed genus depends only on the parity of $a$, this paper contributes to the odd-degree case for genus $4$, whereas our previous paper contributes to the even-degree case.

Superspecial Trigonal Curves of Genus 5

In this paper, we enumerate superspecial trigonal curves of genus 5 over for any natural number a if and for odd a if This paper provides an algorithm enumerating superspecial trigonal curves of genus 5 over for arbitrary q. We implemented the algorithm over computer algebra system Magma. With help of computational results obtained by our implementation, we prove the nonexistence (resp. existence) of superspecial trigonal curves of genus 5 over for q = 7, 72 and 13 (resp. q = 11). Executing the algorithm also determines the number of isomorphism classes of such curves for q = 11 with explicit defining equations.

A fast randomized geometric algorithm for computing Riemann-Roch spaces

We propose a probabilistic variant of Brill-Noether's algorithm for computing a basis of the Riemann-Roch space $L(D)$ associated to a divisor $D$ on a projective nodal plane curve $\mathcal C$ over a sufficiently large perfect field $k$. Our main result shows that this algorithm requires at most $O(\max(\mathrm{deg}(\mathcal C)^{2\omega}, \mathrm{deg}(D_+)^\omega))$ arithmetic operations in $k$, where $\omega$ is a feasible exponent for matrix multiplication and $D_+$ is the smallest effective divisor such that $D_+\geq D$. This improves the best known upper bounds on the complexity of computing Riemann-Roch spaces. Our algorithm may fail, but we show that provided that a few mild assumptions are satisfied, the failure probability is bounded by $O(\max(\mathrm{deg}(\mathcal C)^4, \mathrm{deg}(D_+)^2)/\lvert \mathcal E\rvert)$, where $\mathcal E$ is a finite subset of $k$ in which we pick elements uniformly at random. We provide a freely available C++/NTL implementation of the proposed algorithm and we present experimental data. In particular, our implementation enjoys a speedup larger than 6 on many examples (and larger than 200 on some instances over large finite fields) compared to the reference implementation in the Magma computer algebra system. As a by-product, our algorithm also yields a method for computing the group law on the Jacobian of a smooth plane curve of genus $g$ within $O(g^\omega)$ operations in $k$, which equals the best known complexity for this problem.

Do non-free LCD codes over finite commutative Frobenius rings exist?

In this paper, we clarify some aspects of LCD codes in the literature. We first prove that non-free LCD codes do not exist over finite commutative Frobenius local rings. We then obtain a necessary and sufficient condition for the existence of LCD codes over a finite commutative Frobenius ring. We later show that a free constacyclic code over a finite chain ring is an LCD code if and only if it is reversible, and also provide a necessary and sufficient condition for a constacyclic code to be reversible. We illustrate the minimum Lee distance of LCD codes over some finite commutative chain rings with examples. We found some new optimal cyclic codes over $${\mathbb {Z}}_4$$ of different lengths which are LCD codes using computer algebra system MAGMA.

A new method for recognising Suzuki groups

We present a new algorithm for constructive recognition of the Suzuki groups in their natural representations. The algorithm runs in Las Vegas polynomial time given a discrete logarithm oracle. An implementation is available in the Magma computer algebra system.

Superspecial curves of genus 4 in small characteristic

This paper contains a complete study of superspecial curves of genus 4 in characteristic p≤7. We prove that there does not exist a superspecial curve of genus 4 in characteristic 7. This is a negative answer to the genus 4 case of the problem proposed by Ekedahl [9] in 1987. This implies the non-existence of maximal curve of genus 4 over F49, which updates the table at manypoints.org. We give an algorithm to enumerate superspecial nonhyperelliptic curves in arbitrary p≥5, and for p≤7 we execute it with our implementation on a computer algebra system Magma. Our result in p=5 re-proves the uniqueness of maximal curves of genus 4 over F25, see [11] for the original theoretical proof.In Appendix, we present a general method determining Hasse–Witt matrices of curves which are complete intersections.

A family of surfaces with and Albanese map of degree 3

AbstractWe study a family of surfaces of general type with and , originally constructed by Cancian and Frapporti by using the Computer Algebra System MAGMA. We provide an alternative, computer‐free construction of these surfaces, that allows us to describe their Albanese map and their moduli space.

A fast isomorphism test for groups whose Lie algebra has genus 2

Motivated by the desire for better isomorphism tests for finite groups, we present a polynomial-time algorithm for deciding isomorphism within a class of p-groups that is well-suited to studying local properties of general groups. We also report on the performance of an implementation of the algorithm in the computer algebra system magma.

ALGORITHMS FOR GALOIS EXTENSIONS OF GLOBAL FUNCTION FIELDS

In this thesis we consider the computation of integral closures in cyclic Galois extensions of global function fields and the determination of Galois groups of polynomials over global function fields. The development of methods to efficiently compute integral closures and Galois groups are listed as two of the four most important tasks of number theory considered by Zassenhaus [Poh94]. We describe an algorithm each for computing integral closures specifically for Kummer, Artin–Schreier and Artin–Schreier–Witt extensions. These algorithms are more efficient than previous algorithms because they compute a global (pseudo) basis for such orders, in most cases without using a normal form computation. For Artin–Schreier–Witt extensions where the normal form computation may be necessary we attempt to minimise the number of pseudo generators which are input to the normal form. These integral closure algorithms for cyclic extensions can lead to constructing Goppa codes, which can correct a large proportion of errors, more efficiently. The general algorithm we describe to compute Galois groups is an extension of the algorithm of [FK14] to polynomials over function fields of characteristic p. This algorithm has no restrictions on the degrees of the polynomials it can compute Galois groups for. Previous algorithms have been restricted to polynomials of degree at most 23. Characteristic 2 presents additional challenges as we need to adjust our use of invariants because some invariants do not work in characteristic 2 as they do in other characteristics. We also describe how this algorithm can be used to compute Galois groups of reducible polynomials, including those over function fields of characteristic p. All of the algorithms described in this thesis have been implemented by the author in the Magma Computer Algebra System [CBFS13] and perform effectively as is shown by a number of examples and a collection of timings.

An [formula omitted]-quotient algorithm for finitely presented groups on arbitrarily many generators

We generalize the Plesken–Fabiańska L2-quotient algorithm for finitely presented groups on two or three generators to allow an arbitrary number of generators. The main difficulty lies in a constructive description of the invariant ring of GL(2,K) on m copies of SL(2,K) by simultaneous conjugation. By giving this description, we generalize and simplify some of the known results in invariant theory. An implementation of the algorithm is available in the computer algebra system Magma.

Effective black-box constructive recognition of classical groups

We describe a black-box Las Vegas algorithm to construct standard generators for classical groups defined over finite fields. We assume that the field has size at least 4 and that oracles to solve certain problems are available. Subject to these assumptions, the algorithm runs in polynomial time. A practical implementation of our algorithm is distributed with the computer algebra system Magma.

Recognising the small Ree groups in their natural representations

We present Las Vegas algorithms for constructive recognition and constructive membership testing of the Ree groups G22(q)=Ree(q), where q=32m+1 for some m>0, in their natural representations of degree 7. The input is a generating set X⊂GL(7,q).The constructive recognition algorithm is polynomial time given a discrete logarithm oracle. The constructive membership testing consists of a pre-processing step, that only needs to be executed once for a given X, and a main step. The latter is polynomial time, and the former is polynomial time given a discrete logarithm oracle.Implementations of the algorithms are available for the computer algebra system Magma.

The Weight Distributions of a Class of Cyclic Codes with Three Nonzeros over𝔽3

Cyclic codes have efficient encoding and decoding algorithms. The decoding error probability and the undetected error probability are usually bounded by or given from the weight distributions of the codes. Most researches are about the determination of the weight distributions of cyclic codes with few nonzeros, by using quadratic forms and exponential sums but limited to low moments. In this paper, we focus on the application of higher moments of the exponential sums to determine the weight distributions of a class of ternary cyclic codes with three nonzeros, combining with not only quadratic forms but also MacWilliams’ identities. Another application of this paper is to emphasize the computer algebra system Magma for the investigation of the higher moments. In the end, the result is verified by one example using Matlab.

Fundamental domains for congruence subgroups of [formula omitted] in positive characteristic

In this work, we construct fundamental domains for congruence subgroups of SL 2 ( F q [ t ] ) and PGL 2 ( F q [ t ] ) . Our method uses Gekeler's description of the fundamental domains on the Bruhat–Tits tree X = X q + 1 in terms of cosets of subgroups. We compute the fundamental domains for a number of congruence subgroups explicitly as graphs of groups using the computer algebra system Magma.

Computing Syzygies by Faugère’s F5 algorithm

In this paper, we introduce a new algorithm for computing a set of generators for the syzygies on a sequence of polynomials. For this, we extend a given sequence of polynomials to a Grobner basis using Faugere’s F5 algorithm (A new efficient algorithm for computing Grobner bases without reduction to zero (F5). ISSAC, ACM Press, pp 75–83, 2002). We show then that if we keep all the reductions to zero during this computation, then at termination (by adding principal syzygies) we obtain a basis for the module of syzygies on the input polynomials. We have implemented our algorithm in the computer algebra system Magma, and we evaluate its performance via some examples.