1,067 research outputs found
Lower Bounds for RAMs and Quantifier Elimination
We are considering RAMs , with wordlength , whose arithmetic
instructions are the arithmetic operations multiplication and addition modulo
, the unary function , the binary
functions (with ), ,
, and the boolean vector operations defined on
sequences of length . It also has the other RAM instructions. The size
of the memory is restricted only by the address space, that is, it is
words. The RAMs has a finite instruction set, each instruction is encoded by a
fixed natural number independently of . Therefore a program can run on
each machine , if is sufficiently large. We show that there
exists an and a program , such that it satisfies the following
two conditions.
(i) For all sufficiently large , if running on gets an
input consisting of two words and , then, in constant time, it gives a
output .
(ii) Suppose that is a program such that for each sufficiently large
, if , running on , gets a word of length as an
input, then it decides whether there exists a word of length such that
. Then, for infinitely many positive integers , there exists a
word of length , such that the running time of on at
input is at least
A better upper bound on the number of triangulations of a planar point set
We show that a point set of cardinality in the plane cannot be the vertex
set of more than straight-edge triangulations of its convex
hull. This improves the previous upper bound of .Comment: 6 pages, 1 figur
Almost maximally almost-periodic group topologies determined by T-sequences
A sequence in a group is a {\em -sequence} if there is a
Hausdorff group topology on such that
. In this paper, we provide several
sufficient conditions for a sequence in an abelian group to be a -sequence,
and investigate special sequences in the Pr\"ufer groups
. We show that for , there is a Hausdorff group
topology on that is determined by a -sequence,
which is close to being maximally almost-periodic--in other words, the von
Neumann radical is a non-trivial finite
subgroup. In particular, . We also prove that the
direct sum of any infinite family of finite abelian groups admits a group
topology determined by a -sequence with non-trivial finite von Neumann
radical.Comment: v2 - accepted (discussion on non-abelian case is removed, replaced by
new results on direct sums of finite abelian groups
Datalog vs first-order logic
Our main result is that every datalog query expressible in first-order logic is bounded; in terms of classical model theory it is a kind of compactness theorem for finite structures. In addition, we give some counter-examples delimiting the main result
A new proof of the graph removal lemma
Let H be a fixed graph with h vertices. The graph removal lemma states that
every graph on n vertices with o(n^h) copies of H can be made H-free by
removing o(n^2) edges. We give a new proof which avoids Szemer\'edi's
regularity lemma and gives a better bound. This approach also works to give
improved bounds for the directed and multicolored analogues of the graph
removal lemma. This answers questions of Alon and Gowers.Comment: 17 page
Lower bounds for on-line graph colorings
We propose two strategies for Presenter in on-line graph coloring games. The
first one constructs bipartite graphs and forces any on-line coloring algorithm
to use colors, where is the number of vertices in the
constructed graph. This is best possible up to an additive constant. The second
strategy constructs graphs that contain neither nor as a subgraph
and forces colors. The best known
on-line coloring algorithm for these graphs uses colors
On graphs with a large chromatic number containing no small odd cycles
In this paper, we present the lower bounds for the number of vertices in a
graph with a large chromatic number containing no small odd cycles
Sorting and Selection with Imprecise Comparisons
In experimental psychology, the method of paired comparisons was proposed as a means for ranking preferences amongst n elements of a human subject. The method requires performing all (n2) comparisons then sorting elements according to the number of wins. The large number of comparisons is performed to counter the potentially faulty decision-making of the human subject, who acts as an imprecise comparator.
We consider a simple model of the imprecise comparisons: there exists some δ> 0 such that when a subject is given two elements to compare, if the values of those elements (as perceived by the subject) differ by at least δ, then the comparison will be made correctly; when the two elements have values that are within δ, the outcome of the comparison is unpredictable. This δ corresponds to the just noticeable difference unit (JND) or difference threshold in the psychophysics literature, but does not require the statistical assumptions used to define this value.
In this model, the standard method of paired comparisons minimizes the errors introduced by the imprecise comparisons at the cost of (n2) comparisons. We show that the same optimal guarantees can be achieved using 4 n 3/2 comparisons, and we prove the optimality of our method. We then explore the general tradeoff between the guarantees on the error that can be made and number of comparisons for the problems of sorting, max-finding, and selection. Our results provide close-to-optimal solutions for each of these problems.Engineering and Applied Science
Characteristics of Glial Reaction in the Perinatal Rat Cortex: Effect of Lesion Size in the ‘Critical Period’
In this study we investigate the capability of
lesions, performed between embryonic day E18
and postnatal day P6, to provoke glial reaction.
Two different lesion types were applied: ‘severe’
lesion (tissue defect) and ’light’ lesion (stab
wound). The glial reaction was detected with
immunostain[ng against glial fibrillary acidic
protein. When performed as early as P0, severe
lesions could result in reactive gliosis, which
persisted even after a month. The glial reaction
was detected at P6/P7 and became strong by P8,
regardless of the age when the animals were
lesioned between P0 and P5. Namely, a strict
limit could be estimated for the age when
reactive glia were already found rather than for
the age when glial reaction-provoking lesions
could occur. After prenatal lesions, no glial
reaction developed, but the usual glia limitans
covered the deformed brain, surface. Light
lesions provoked glial reactions when
performed at P6. In conclusion, three scenarios
were found, depending on the age of the animal
at injury: (i) healing without glial reaction,
regardless of the remaining deformation; (ii) depending on the size of the lesion, either healing without residuum or with remaining
tissue defect plus reactive gliosis; and (iii) healing always with reactive gliosis. The age limits between them were at P0 and P5. The glial reactivity seemingly appears after the end of the neuronal migration and just precedes the massive transformation of the radial glia into astrocytes. Estimating the position of the appearance of glial reactivity among the events of cortical maturation can help to adapt the experimental results to humans
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