6,233 research outputs found
A survey of clones on infinite sets
A clone on a set X is a set of finitary operations on X which contains all
projections and which is moreover closed under functional composition. Ordering
all clones on X by inclusion, one obtains a complete algebraic lattice, called
the clone lattice. We summarize what we know about the clone lattice on an
infinite base set X and formulate what we consider the most important open
problems.Comment: 37 page
A matroid extension result
Adding elements to matroids can be fraught with difficulty. In the V\'amos
matroid , there are four independent sets and such
that is a -separation while exactly three of
the local connectivities , ,
, and are one, with the fourth being
zero. As is well known, there is no extension of by a non-loop element
such that is a circuit for all . This paper proves that a
matroid can be extended by a fixed element in the guts of a -separation
provided no V\'amos-like structure is present
Extremal problems on shadows and hypercuts in simplicial complexes
Let be an -vertex forest. We say that an edge is in the
shadow of if contains a cycle. It is easy to see that if
is "almost a tree", that is, it has edges, then at least
edges are in its shadow and this is tight.
Equivalently, the largest number of edges an -vertex cut can have is
. These notions have natural analogs in higher
-dimensional simplicial complexes, graphs being the case . The results
in dimension turn out to be remarkably different from the case in graphs.
In particular the corresponding bounds depend on the underlying field of
coefficients. We find the (tight) analogous theorems for . We construct
-dimensional "-almost-hypertrees" (defined below) with an empty
shadow. We also show that the shadow of an "-almost-hypertree"
cannot be empty, and its least possible density is . In
addition we construct very large hyperforests with a shadow that is empty over
every field.
For even, we construct -dimensional -almost-hypertree whose shadow has density .
Finally, we mention several intriguing open questions
Algebraic recognizability of regular tree languages
We propose a new algebraic framework to discuss and classify recognizable
tree languages, and to characterize interesting classes of such languages. Our
algebraic tool, called preclones, encompasses the classical notion of syntactic
Sigma-algebra or minimal tree automaton, but adds new expressivity to it. The
main result in this paper is a variety theorem \`{a} la Eilenberg, but we also
discuss important examples of logically defined classes of recognizable tree
languages, whose characterization and decidability was established in recent
papers (by Benedikt and S\'{e}goufin, and by Bojanczyk and Walukiewicz) and can
be naturally formulated in terms of pseudovarieties of preclones. Finally, this
paper constitutes the foundation for another paper by the same authors, where
first-order definable tree languages receive an algebraic characterization
Effects on the transcriptome upon deletion of a distal element cannot be predicted by the size of the H3K27Ac peak in human cells.
Genome-wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs) associated with increased risk for colorectal cancer (CRC). A molecular understanding of the functional consequences of this genetic variation is complicated because most GWAS SNPs are located in non-coding regions. We used epigenomic information to identify H3K27Ac peaks in HCT116 colon cancer cells that harbor SNPs associated with an increased risk for CRC. Employing CRISPR/Cas9 nucleases, we deleted a CRC risk-associated H3K27Ac peak from HCT116 cells and observed large-scale changes in gene expression, resulting in decreased expression of many nearby genes. As a comparison, we showed that deletion of a robust H3K27Ac peak not associated with CRC had minimal effects on the transcriptome. Interestingly, although there is no H3K27Ac peak in HEK293 cells in the E7 region, deletion of this region in HEK293 cells decreased expression of several of the same genes that were downregulated in HCT116 cells, including the MYC oncogene. Accordingly, deletion of E7 causes changes in cell culture assays in HCT116 and HEK293 cells. In summary, we show that effects on the transcriptome upon deletion of a distal regulatory element cannot be predicted by the size or presence of an H3K27Ac peak
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