2,043 research outputs found
Boolean Dependence Logic and Partially-Ordered Connectives
We introduce a new variant of dependence logic called Boolean dependence
logic. In Boolean dependence logic dependence atoms are of the type
=(x_1,...,x_n,\alpha), where \alpha is a Boolean variable. Intuitively, with
Boolean dependence atoms one can express quantification of relations, while
standard dependence atoms express quantification over functions.
We compare the expressive power of Boolean dependence logic to dependence
logic and first-order logic enriched by partially-ordered connectives. We show
that the expressive power of Boolean dependence logic and dependence logic
coincide. We define natural syntactic fragments of Boolean dependence logic and
show that they coincide with the corresponding fragments of first-order logic
enriched by partially-ordered connectives with respect to expressive power. We
then show that the fragments form a strict hierarchy.Comment: 41 page
Random and externally controlled occurrences of Dansgaard-Oeschger events
Dansgaard–Oeschger (DO) events constitute the most pronounced mode of
centennial to millennial climate variability of the last glacial period.
Since their discovery, many decades of research have been devoted to
understand the origin and nature of these rapid climate shifts. In recent
years, a number of studies have appeared that report emergence of DO-type
variability in fully coupled general circulation models via different
mechanisms. These mechanisms result in the occurrence of DO events at varying
degrees of regularity, ranging from periodic to random. When examining the
full sequence of DO events as captured in the North Greenland Ice Core
Project (NGRIP) ice core record, one can observe high irregularity in the
timing of individual events at any stage within the last glacial period. In
addition to the prevailing irregularity, certain properties of the DO event
sequence, such as the average event frequency or the relative distribution of
cold versus warm periods, appear to be changing throughout the glacial. By
using statistical hypothesis tests on simple event models, we investigate
whether the observed event sequence may have been generated by stationary
random processes or rather was strongly modulated by external factors. We
find that the sequence of DO warming events is consistent with a stationary
random process, whereas dividing the event sequence into warming and cooling
events leads to inconsistency with two independent event processes. As we
include external forcing, we find a particularly good fit to the observed DO
sequence in a model where the average residence time in warm periods are
controlled by global ice volume and cold periods by boreal summer insolation
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