402,524 research outputs found
Partially defined computer instructions and guards
AbstractWe here extend our earlier work on the theory of computer instructions to consider instructions which are only partially defined. For every such instruction, we assume that it is defined whenever a certain Boolean expression is true; we refer to such a Boolean expression as a guard, following Dijkstra. This is a special case of a more general function on the set of states of a computer, representing an expression in a programming language. Many constructs for instructions now generalize to partially defined instructions; in particular, we define the notion of conditional input and output regions, as well as the relevant region of a more general expression. Fundamental theorems about instructions generalize to theorems about guards and about partially defined instructions. We also define the parallel execution of such instructions, which is useful in validating a generalized instruction commutativity criterion
User's guide for a flat wake rotor inflow/wake velocity prediction code, DOWN
A computer code named DOWN was created to implement a flat wake theory for the calculation of rotor inflow and wake velocities. A brief description of the code methodology and instructions for its use are given. The code will be available from NASA's Computer Software Management and Information Center (COSMIC)
EAC: A program for the error analysis of STAGS results for plates
A computer code is now available for estimating the error in results from the STAGS finite element code for a shell unit consisting of a rectangular orthotropic plate. This memorandum contains basic information about the computer code EAC (Error Analysis and Correction) and describes the connection between the input data for the STAGS shell units and the input data necessary to run the error analysis code. The STAGS code returns a set of nodal displacements and a discrete set of stress resultants; the EAC code returns a continuous solution for displacements and stress resultants. The continuous solution is defined by a set of generalized coordinates computed in EAC. The theory and the assumptions that determine the continuous solution are also outlined in this memorandum. An example of application of the code is presented and instructions on its usage on the Cyber and the VAX machines have been provided
Two dimensional aerodynamic interference effects on oscillating airfoils with flaps in ventilated subsonic wind tunnels
The numerical computation of unsteady airloads acting upon thin airfoils with multiple leading and trailing-edge controls in two-dimensional ventilated subsonic wind tunnels is studied. The foundation of the computational method is strengthened with a new and more powerful mathematical existence and convergence theory for solving Cauchy singular integral equations of the first kind, and the method of convergence acceleration by extrapolation to the limit is introduced to analyze airfoils with flaps. New results are presented for steady and unsteady flow, including the effect of acoustic resonance between ventilated wind-tunnel walls and airfoils with oscillating flaps. The computer program TWODI is available for general use and a complete set of instructions is provided
Causality, Information and Biological Computation: An algorithmic software approach to life, disease and the immune system
Biology has taken strong steps towards becoming a computer science aiming at
reprogramming nature after the realisation that nature herself has reprogrammed
organisms by harnessing the power of natural selection and the digital
prescriptive nature of replicating DNA. Here we further unpack ideas related to
computability, algorithmic information theory and software engineering, in the
context of the extent to which biology can be (re)programmed, and with how we
may go about doing so in a more systematic way with all the tools and concepts
offered by theoretical computer science in a translation exercise from
computing to molecular biology and back. These concepts provide a means to a
hierarchical organization thereby blurring previously clear-cut lines between
concepts like matter and life, or between tumour types that are otherwise taken
as different and may not have however a different cause. This does not diminish
the properties of life or make its components and functions less interesting.
On the contrary, this approach makes for a more encompassing and integrated
view of nature, one that subsumes observer and observed within the same system,
and can generate new perspectives and tools with which to view complex diseases
like cancer, approaching them afresh from a software-engineering viewpoint that
casts evolution in the role of programmer, cells as computing machines, DNA and
genes as instructions and computer programs, viruses as hacking devices, the
immune system as a software debugging tool, and diseases as an
information-theoretic battlefield where all these forces deploy. We show how
information theory and algorithmic programming may explain fundamental
mechanisms of life and death.Comment: 30 pages, 8 figures. Invited chapter contribution to Information and
Causality: From Matter to Life. Sara I. Walker, Paul C.W. Davies and George
Ellis (eds.), Cambridge University Pres
The COREL and W12SC3 computer programs for supersonic wing design and analysis
Two computer codes useful in the supersonic aerodynamic design of wings, including the supersonic maneuver case are described. The nonlinear full potential equation COREL code performs an analysis of a spanwise section of the wing in the crossflow plane by assuming conical flow over the section. A subsequent approximate correction to the solution can be made in order to account for nonconical effects. In COREL, the flow-field is assumed to be irrotional (Mach numbers normal to shock waves less than about 1.3) and the full potential equation is solved to obtain detailed results for the leading edge expansion, supercritical crossflow, and any crossflow shockwaves. W12SC3 is a linear theory panel method which combines and extends elements of several of Woodward's codes, with emphasis on fighter applications. After a brief review of the aerodynamic theory used by each method, the use of the codes is illustrated with several examples, detailed input instructions and a sample case
Dynamical Systems that Heal
Malware plays a key role in attacking critical infrastructure. With this problem in mind, we introduce systems that heal from a broader perspective than the standard digital computer model: Our goal in a more general theory is to be applicable to systems that contain subsystems that do not solely rely on the execution of register machine instructions. Our broader approach assumes a dynamical system that performs tasks. Our primary contribution defines a principle of self-modifiability in dynamical systems and demonstrates how it can be used to heal a malfunctioning dynamical system. As far as we know, to date there has not been a mathematical notion of self-modifiability in dynamical systems; hitherto there has not been a formal system for describing how to heal damaged computer instructions or to heal differential equations that perform tasks
Program algebra for Turing-machine programs
This paper presents an algebraic theory of instruction sequences with
instructions for Turing tapes as basic instructions, the behaviours produced by
the instruction sequences concerned under execution, and the interaction
between such behaviours and Turing tapes provided by an execution environment.
This theory provides a setting for the development of theory in areas such as
computability and computational complexity that distinguishes itself by
offering the possibility of equational reasoning and being more general than
the setting provided by a known version of the Turing-machine model of
computation. The theory is essentially an instantiation of a parameterized
algebraic theory which is the basis of a line of research in which issues
relating to a wide variety of subjects from computer science have been
rigorously investigated thinking in terms of instruction sequences.Comment: 19 pages, Sect. 2--4 are largely shortened versions of Sect. 2--4 of
arXiv:1808.04264, which, in turn, draw from preliminary sections of several
earlier papers; 21 pages, some remarks in Sect.1 and Sect.10 adde
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