2,860 research outputs found
Doubles and Negatives are Positive (in Self-Assembly)
In the abstract Tile Assembly Model (aTAM), the phenomenon of cooperation
occurs when the attachment of a new tile to a growing assembly requires it to
bind to more than one tile already in the assembly. Often referred to as
``temperature-2'' systems, those which employ cooperation are known to be quite
powerful (i.e. they are computationally universal and can build an enormous
variety of shapes and structures). Conversely, aTAM systems which do not
enforce cooperative behavior, a.k.a. ``temperature-1'' systems, are conjectured
to be relatively very weak, likely to be unable to perform complex computations
or algorithmically direct the process of self-assembly. Nonetheless, a variety
of models based on slight modifications to the aTAM have been developed in
which temperature-1 systems are in fact capable of Turing universal computation
through a restricted notion of cooperation. Despite that power, though, several
of those models have previously been proven to be unable to perform or simulate
the stronger form of cooperation exhibited by temperature-2 aTAM systems.
In this paper, we first prove that another model in which temperature-1
systems are computationally universal, namely the restricted glue TAM (rgTAM)
in which tiles are allowed to have edges which exhibit repulsive forces, is
also unable to simulate the strongly cooperative behavior of the temperature-2
aTAM. We then show that by combining the properties of two such models, the
Dupled Tile Assembly Model (DTAM) and the rgTAM into the DrgTAM, we derive a
model which is actually more powerful at temperature-1 than the aTAM at
temperature-2. Specifically, the DrgTAM, at temperature-1, can simulate any
aTAM system of any temperature, and it also contains systems which cannot be
simulated by any system in the aTAM
Apollo Spacecraft Systems Analysis Program. Analysis of Rendezvous Radar Pearl Flight Test Data
Flight test data analysis for rendezvous radar performance during simulated lunar missio
Reflections on Tiles (in Self-Assembly)
We define the Reflexive Tile Assembly Model (RTAM), which is obtained from
the abstract Tile Assembly Model (aTAM) by allowing tiles to reflect across
their horizontal and/or vertical axes. We show that the class of directed
temperature-1 RTAM systems is not computationally universal, which is
conjectured but unproven for the aTAM, and like the aTAM, the RTAM is
computationally universal at temperature 2. We then show that at temperature 1,
when starting from a single tile seed, the RTAM is capable of assembling n x n
squares for n odd using only n tile types, but incapable of assembling n x n
squares for n even. Moreover, we show that n is a lower bound on the number of
tile types needed to assemble n x n squares for n odd in the temperature-1
RTAM. The conjectured lower bound for temperature-1 aTAM systems is 2n-1.
Finally, we give preliminary results toward the classification of which finite
connected shapes in Z^2 can be assembled (strictly or weakly) by a singly
seeded (i.e. seed of size 1) RTAM system, including a complete classification
of which finite connected shapes be strictly assembled by a "mismatch-free"
singly seeded RTAM system.Comment: New results which classify the types of shapes which can
self-assemble in the RTAM have been adde
Women at Risk: Why Many Women Are Forgoing Needed Health Care
Based on Commonwealth Fund 2007 Biennial Health Insurance Survey data, compares women's rates of uninsurance or underinsurance, sources of coverage, out-of-pocket and premium expenses, access to care, medical debt, and unmet needs, with those of men
Realizing Health Reform's Potential: Women and the Affordable Care Act of 2010
Outlines the 2010 healthcare reform provisions that will benefit women, including subsidized and improved coverage and bans on lifetime caps, rescissions, and rating on gender. Analyzes how each will address women's growing exposure to healthcare costs
Astrochemical confirmation of the rapid evolution of massive YSOs and explanation for the inferred ages of hot cores
Aims. To understand the roles of infall and protostellar evolution on the
envelopes of massive young stellar objects (YSOs).
Methods. The chemical evolution of gas and dust is traced, including infall
and realistic source evolution. The temperatures are determined
self-consistently. Both ad/desorption of ices using recent laboratory
temperature-programmed-desorption measurements are included.
Results. The observed water abundance jump near 100 K is reproduced by an
evaporation front which moves outward as the luminosity increases. Ion-molecule
reactions produce water below 100 K. The age of the source is constrained to t
\~ 8 +/- 4 x 10^4 yrs since YSO formation. It is shown that the chemical
age-dating of hot cores at ~ few x 10^3 - 10^4 yr and the disappearance of hot
cores on a timescale of ~ 10^5 yr is a natural consequence of infall in a
dynamic envelope and protostellar evolution. Dynamical structures of ~ 350AU
such as disks should contain most of the complex second generation species. The
assumed order of desorption kinetics does not affect these results.Comment: Accepted by A&A Letters; 4 pages, 5 figure
Negative Interactions in Irreversible Self-Assembly
This paper explores the use of negative (i.e., repulsive) interaction the
abstract Tile Assembly Model defined by Winfree. Winfree postulated negative
interactions to be physically plausible in his Ph.D. thesis, and Reif, Sahu,
and Yin explored their power in the context of reversible attachment
operations. We explore the power of negative interactions with irreversible
attachments, and we achieve two main results. Our first result is an
impossibility theorem: after t steps of assembly, Omega(t) tiles will be
forever bound to an assembly, unable to detach. Thus negative glue strengths do
not afford unlimited power to reuse tiles. Our second result is a positive one:
we construct a set of tiles that can simulate a Turing machine with space bound
s and time bound t, while ensuring that no intermediate assembly grows larger
than O(s), rather than O(s * t) as required by the standard Turing machine
simulation with tiles
Maintaining Health Insurance During a Recession: Likely COBRA Eligibility
Assesses laid-off workers' eligibility and financial ability to extend employer-sponsored insurance through COBRA. Recommends extending COBRA and providing premium assistance, as well as expanding Medicaid and State Children's Health Insurance Programs
- …