A Modular Integer GCD Algorithm

This paper describes the first algorithm to compute the greatest common divisor (GCD) of two n-bit integers using a modular representation for intermediate values U, V and also for the result. It is based on a reduction step, similar to one used in the accelerated algorithm [T. Jebelean, A generalization of the binary GCD algorithm, in: ISSAC \u2793: International Symposium on Symbolic and Algebraic Computation, Kiev, Ukraine, 1993, pp. 111–116; K. Weber, The accelerated integer GCD algorithm, ACM Trans. Math. Softw. 21 (1995) 111–122] when U and V are close to the same size, that replaces U by (U-bV)/p, where p is one of the prime moduli and b is the unique integer in the interval (-p/2,p/2) such that b=UV ^-1(mod p) . When the algorithm is executed on a bit common CRCW PRAM with O(n log n log log log n) processors, it takes O(n) time in the worst case. A heuristic model of the average case yields O(n/log n) time on the same number of processors

Improvements on the accelerated integer GCD algorithm

6 pagesInternational audienceThe present paper analyses and presents several improvements to the algorithm for finding the $(a,b)$-pairs of integers used in the $k$-ary reduction of the right-shift $k$-ary integer GCD algorithm. While the worst-case complexity of Weber's ''Accelerated integer GCD algorithm'' is \cO\l(\log_\phi(k)^2\r), we show that the worst-case number of iterations of the while loop is exactly \tfrac 12 \l\lfloor \log_{\phi}(k)\r\rfloor, where \phi := \tfrac 12 \l(1+\sqrt{5}\r).\par We suggest improvements on the average complexity of the latter algorithm and also present two new faster residual algorithms: the sequential and the parallel one. A lower bound on the probability of avoiding the while loop in our parallel residual algorithm is also given

Parallel Implementation of the Accelerated Integer GCD Algorithm

AbstractThe accelerated integer greatest common divisor (GCD) algorithm has been shown to be one of the most efficient in practice. This paper describes a parallel implementation of the accelerated algorithm for the Sequent Balance, a shared-memory multiprocessor. For input of roughly 10 000 digits, it displays speed-ups of 1.6, 2.5, 3.4 and 4.0 using 2, 4, 8 and 16 processors, respectively

An upper bound for the genus of a curve without points of small degree

In this paper I prove that for any prime $p$ there is a constant $C_p>0$ such that for any $n>0$ and for any $p$-power $q$ there is a smooth, projective, absolutely irreducible curve over $\mathbb{F}_q$ of genus $g\leq C_p q^n$ without points of degree smaller than $n$.Comment: This is part of a Phd thesis at Universit\`a 'Sapienza' of Rom

Efficient Algorithms for gcd and Cubic Residuosity in the Ring of Eisenstein Integers

We present simple and efficient algorithms for computing gcd and cubic residuosity in the ring of Eisenstein integers, Z[zeta] , i.e. the integers extended with zeta , a complex primitive third root of unity. The algorithms are similar and may be seen as generalisations of the binary integer gcd and derived Jacobi symbol algorithms. Our algorithms take time O(n^2) for n bit input. This is an improvement from the known results based on the Euclidian algorithm, and taking time O(n· M(n)), where M(n) denotes the complexity of multiplying n bit integers. The new algorithms have applications in practical primality tests and the implementation of cryptographic protocols. The technique underlying our algorithms can be used to obtain equally fast algorithms for gcd and quartic residuosity in the ring of Gaussian integers, Z[i]