# Numero 35

fredag 15 november 1996, årgång 26
*Veckoblad om forskning, undervisning och administration m.m.*

NADA - Institutionen för numerisk analys och datalogi, KTH

## Kalendarium

Aktuellt kalendarium (**inte**
kalendariet som hör specifikt till denna Numeroutgåva).

# Teoretisk datalogi-seminarium: Parallel repetition

Alex Russell, NADA, håller seminarium i teoretisk datalogi
under ovanstående rubrik på torsdag (21 nov.) kl. 15.15-16.15
i seminarierum 1537 (Lindstedtsv. 3, plan 5).

## Abstract

The notion of a two-prover proof system, introduced by Ben-Or,
et. al. in 1988, is a cornerstone in the remarkably successful
theory of interactive and probabilistically checkable proofs.
Although this theory provides startling new models for many
known complexity classes, most celebrated is it's unexpected
connection to approximation algorithms which has provided the
first general framework for proving lower bounds on
approximability.
The behavior of two-prover proof systems under "parallel
repetition", a natural method for amplifying such systems,
remained poorly understood until 1995 when Ran Raz fairly
closed the area.

This talk has two goals. First, we shall introduce two-prover
proof systems and give some indication of their connection
with approximation lower bounds. Second, we shall discuss the
ramifications of parallel repetition for these applications
and survey Raz's proof the parallel repetition theorem.

Johan Håstad

# SANS-seminarium: connectionist modelling of generalization in conditioning

Christian Balkenius, Dept. of Cognitive Science, Lunds
Universitet, håller SANS-seminarium under ovanstående rubrik
på tisdag (19 nov.) kl. 13.15-14.30 i seminarierum 4523
(Lindstedtsv. 5, plan 5).

## Abstract

While it was once believed that conditioning in some form
could account for all learning it is now widely recognized
that this is not the case. Many types of learning does not
fall into the simple pairing of a neutral stimulus with a
subsequent significant event. There are however a number of
learning situations which can be adequately described as
instances of conditioning, and any complete model of learning
must include mechanisms which can handle such learning.
However, the simplicity of the experimental procedure does not
mean that the processes and representations required for
conditioning are simple too.
I will describe my research on how a representation at
multiple scales can be used for generalization in
conditioning. The use of such representations is an efficient
way to code similarities and differences within a stimulus
dimension, and allows a learning system to generalize easily
between various situations. A neural network based model of
classical and instrumental conditioning is presented and its
ability to generalize using multi-scale representations is
demonstrated in a number of simulations.

Mikael Djurfeldt, Anders Lansner

# I korthet

**Disputation**: Techniques for Efficient
Simulation of Computer Architectures, Luis J. Barriga C., IT,
KTH. Torsdagen den 5 december kl. 14.00 i Kollegiesalen,
administrationshuset (Valhallavägen 79). *Numeropärmen
96.141*
**Disputation**: Data-Parallel Implementation of
Prolog, Johan Bevemyr. Onsdagen den 20 november kl. 14.00 i
rum 311, hus 1, Polacksbacken, Computing Science Department,
Uppsala Universitet. *Numeropärmen 96.142*

# PDC-seminarium: Transverse velocity increments in turbulent flow using the RELIEF technique

Uriel Frisch, Observatoire de Nice, Frankrike, håller
PDC-seminarium under ovanstående rubrik på tisdag (19 nov.)
kl. 15.15 i seminarierum 1537 (Lindstedtsv. 3, plan 5).

## Abstract

Nonintrusive measurements of the streamwise velocity of
turbulent: round jets in air are performed by recording
short-time displacements and distorsions of very thin tagging
lines written spanwise into the flow. The lines are written by
Raman Exciting oxygen molecules and are interrogated by Laser
Induced Fluorescence (RELIEF). This gives access to the
spatial structure of transverse velocity increments without
recourse to the Taylor hypothesis. The resolution is
*$25mu$*, comparable to the Kolmogorov scale
*$eta$* for the experiments performed (with
*$R_lambda pprox 300--500$*).
The technique is validated by comparison with results obtained
from other techniques for longitudinal or transverse structure
functions up to order eight. The agreement is consistent with
the estimated errors -- about 5 % on exponents determined by
Extended-Self-Similarity -- and indicates significant
departures from Kolmogorov 1941 scaling.

Probability distribution functions of velocity increments
*$Delta u$* over separations down to *$1.5 eta$*
are reported for the first time. Violent events with
*$Delta u$* comparable to the rms turbulent velocity
fluctuation, are found to take place with statistically
significant probabilities. The shapes of the corresponding
lines suggest the effect of intense slender vortex filaments.

This talk is based on a joint paper by Noullez, Wallace,
Lempert, Miles and Frisch, submitted to Journal of Fluid
Mechanics.

Erik Aurell

Upp till *Numeros hemsida*.

Senast ändrad 14 nov. 1996
<numero@nada.kth.se>