3,891 research outputs found
Perfect Fluids and Bad Metals: Transport Analogies Between Ultracold Fermi Gases and High Superconductors
In this paper, we examine in a unified fashion dissipative transport in
strongly correlated systems. We thereby demonstrate the connection between "bad
metals" (such as the high temperature superconductors) and "perfect fluids"
(such as the ultracold Fermi gases, near unitarity). One motivation of this
work is to communicate to the high energy physics community some of the central
unsolved problems in high superconductors. Because of interest in the
nearly perfect fluidity of the cold gases and because of new tools such as the
AdS/CFT correspondence, this better communication may lead to important
progress in a variety of different fields. A second motivation is to draw
attention to the great power of transport measurements which more directly
reflect the excitation spectrum than, say, thermodynamics and thus strongly
constrain microscopic theories of correlated fermionic superfluids. Our
calculations show that bad metal and perfect fluid behavior is associated with
the presence of a normal state excitation gap which suppresses the effective
number of carriers leading to anomalously low conductivity and viscosity above
the transition temperature . Below we demonstrate that the
condensate collective modes ("phonons") do not couple to transverse probes such
as the shear viscosity. As a result, our calculated shear viscosity at low
becomes arbitrarily small as observed in experiments. In both homogeneous and
trap calculations we do not find the upturn in or (where is
the entropy density) found in most theories. In the process of these studies we
demonstrate compatibility with the transverse sum rule and find reasonable
agreement with both viscosity and cuprate conductivity experiments.Comment: 21 pages, 11 figure
Faster Mutation Analysis via Equivalence Modulo States
Mutation analysis has many applications, such as asserting the quality of
test suites and localizing faults. One important bottleneck of mutation
analysis is scalability. The latest work explores the possibility of reducing
the redundant execution via split-stream execution. However, split-stream
execution is only able to remove redundant execution before the first mutated
statement.
In this paper we try to also reduce some of the redundant execution after the
execution of the first mutated statement. We observe that, although many
mutated statements are not equivalent, the execution result of those mutated
statements may still be equivalent to the result of the original statement. In
other words, the statements are equivalent modulo the current state.
In this paper we propose a fast mutation analysis approach, AccMut. AccMut
automatically detects the equivalence modulo states among a statement and its
mutations, then groups the statements into equivalence classes modulo states,
and uses only one process to represent each class. In this way, we can
significantly reduce the number of split processes. Our experiments show that
our approach can further accelerate mutation analysis on top of split-stream
execution with a speedup of 2.56x on average.Comment: Submitted to conferenc
Automated Refactoring of Nested-IF Formulae in Spreadsheets
Spreadsheets are the most popular end-user programming software, where
formulae act like programs and also have smells. One well recognized common
smell of spreadsheet formulae is nest-IF expressions, which have low
readability and high cognitive cost for users, and are error-prone during reuse
or maintenance. However, end users usually lack essential programming language
knowledge and skills to tackle or even realize the problem. The previous
research work has made very initial attempts in this aspect, while no effective
and automated approach is currently available.
This paper firstly proposes an AST-based automated approach to systematically
refactoring nest-IF formulae. The general idea is two-fold. First, we detect
and remove logic redundancy on the AST. Second, we identify higher-level
semantics that have been fragmented and scattered, and reassemble the syntax
using concise built-in functions. A comprehensive evaluation has been conducted
against a real-world spreadsheet corpus, which is collected in a leading IT
company for research purpose. The results with over 68,000 spreadsheets with 27
million nest-IF formulae reveal that our approach is able to relieve the smell
of over 99\% of nest-IF formulae. Over 50% of the refactorings have reduced
nesting levels of the nest-IFs by more than a half. In addition, a survey
involving 49 participants indicates that for most cases the participants prefer
the refactored formulae, and agree on that such automated refactoring approach
is necessary and helpful
Empirical Evaluation of Test Coverage for Functional Programs
The correlation between test coverage and test effectiveness is important to justify the use of coverage in practice. Existing results on imperative programs mostly show that test coverage predicates effectiveness. However, since functional programs are usually structurally different from imperative ones, it is unclear whether the same result may be derived and coverage can be used as a prediction of effectiveness on functional programs. In this paper we report the first empirical study on the correlation between test coverage and test effectiveness on functional programs. We consider four types of coverage: as input coverages, statement/branch coverage and expression coverage, and as oracle coverages, count of assertions and checked coverage. We also consider two types of effectiveness: raw effectiveness and normalized effectiveness. Our results are twofold. (1) In general the findings on imperative programs still hold on functional programs, warranting the use of coverage in practice. (2) On specific coverage criteria, the results may be unexpected or different from the imperative ones, calling for further studies on functional programs
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