384 research outputs found
Finding Octonionic Eigenvectors Using Mathematica
The eigenvalue problem for 3x3 octonionic Hermitian matrices contains some
surprises, which we have reported elsewhere. In particular, the eigenvalues
need not be real, there are 6 rather than 3 real eigenvalues, and the
corresponding eigenvectors are not orthogonal in the usual sense. The
nonassociativity of the octonions makes computations tricky, and all of these
results were first obtained via brute force (but exact) Mathematica
computations. Some of them, such as the computation of real eigenvalues, have
subsequently been implemented more elegantly; others have not. We describe here
the use of Mathematica in analyzing this problem, and in particular its use in
proving a generalized orthogonality property for which no other proof is known.Comment: LaTeX2e, 22 pages, 8 PS figures (uses included PS prolog; needs
elsart.cls and one of epsffig, epsf, graphicx
A quantitative probabilistic investigation into the accumulation of rounding errors in numerical ODE solution.
We examine numerical rounding errors of some deterministic solvers for systems of ordinary differential equations (ODEs) from a probabilistic viewpoint. We show that the accumulation of rounding errors results in a solution which is inherently random and we obtain the theoretical distribution of the trajectory as a function of time, the step size and the numerical precision of the computer. We consider, in particular, systems which amplify the effect of the rounding errors so that over long time periods the solutions exhibit divergent behaviour. By performing multiple repetitions with different values of the time step size, we observe numerically the random distributions predicted theoretically. We mainly focus on the explicit Euler and fourth order RungeāKutta methods but also briefly consider more complex algorithms such as the implicit solvers VODE and RADAU5 in order to demonstrate that the observed effects are not specific to a particular method
Parikh Image of Pushdown Automata
We compare pushdown automata (PDAs for short) against other representations.
First, we show that there is a family of PDAs over a unary alphabet with
states and stack symbols that accepts one single long word for
which every equivalent context-free grammar needs
variables. This family shows that the classical algorithm for converting a PDA
to an equivalent context-free grammar is optimal even when the alphabet is
unary. Moreover, we observe that language equivalence and Parikh equivalence,
which ignores the ordering between symbols, coincide for this family. We
conclude that, when assuming this weaker equivalence, the conversion algorithm
is also optimal. Second, Parikh's theorem motivates the comparison of PDAs
against finite state automata. In particular, the same family of unary PDAs
gives a lower bound on the number of states of every Parikh-equivalent finite
state automaton. Finally, we look into the case of unary deterministic PDAs. We
show a new construction converting a unary deterministic PDA into an equivalent
context-free grammar that achieves best known bounds.Comment: 17 pages, 2 figure
A week-end off: the first extensive number-theoretical computation on the ENIAC
The first extensive number-theoretical computation run on the ENIAC, is reconstructed. The problem, computing the exponent of 2 modulo a prime, was set up on the ENIAC during a week-end in July 1946 by the number-theorist D.H. Lehmer, with help from his wife Emma and John Mauchly. Important aspects of the ENIAC's design are presented-and the reconstruction of the implementation of the problem on the ENIAC is discussed in its salient points
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Timing, Sequencing and Accumulation of Risk Factors Among Currently Incarcerated Men: Evidence of Developmental Cascades
Parikh's Theorem: A simple and direct automaton construction
Parikh's theorem states that the Parikh image of a context-free language is
semilinear or, equivalently, that every context-free language has the same
Parikh image as some regular language. We present a very simple construction
that, given a context-free grammar, produces a finite automaton recognizing
such a regular language.Comment: 12 pages, 3 figure
Hypercomplex quantum mechanics
The fundamental axioms of the quantum theory do not explicitly identify the
algebraic structure of the linear space for which orthogonal subspaces
correspond to the propositions (equivalence classes of physical questions). The
projective geometry of the weakly modular orthocomplemented lattice of
propositions may be imbedded in a complex Hilbert space; this is the structure
which has traditionally been used. This paper reviews some work which has been
devoted to generalizing the target space of this imbedding to Hilbert modules
of a more general type. In particular, detailed discussion is given of the
simplest generalization of the complex Hilbert space, that of the quaternion
Hilbert module.Comment: Plain Tex, 11 page
How Ordinary Elimination Became Gaussian Elimination
Newton, in notes that he would rather not have seen published, described a
process for solving simultaneous equations that later authors applied
specifically to linear equations. This method that Euler did not recommend,
that Legendre called "ordinary," and that Gauss called "common" - is now named
after Gauss: "Gaussian" elimination. Gauss's name became associated with
elimination through the adoption, by professional computers, of a specialized
notation that Gauss devised for his own least squares calculations. The
notation allowed elimination to be viewed as a sequence of arithmetic
operations that were repeatedly optimized for hand computing and eventually
were described by matrices.Comment: 56 pages, 21 figures, 1 tabl
On Measuring Non-Recursive Trade-Offs
We investigate the phenomenon of non-recursive trade-offs between
descriptional systems in an abstract fashion. We aim at categorizing
non-recursive trade-offs by bounds on their growth rate, and show how to deduce
such bounds in general. We also identify criteria which, in the spirit of
abstract language theory, allow us to deduce non-recursive tradeoffs from
effective closure properties of language families on the one hand, and
differences in the decidability status of basic decision problems on the other.
We develop a qualitative classification of non-recursive trade-offs in order to
obtain a better understanding of this very fundamental behaviour of
descriptional systems
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