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Pattern avoidance in ascent sequences  [PDF]
Paul Duncan,Einar Steingrimsson
Mathematics , 2011,
Abstract: Ascent sequences are sequences of nonnegative integers with restrictions on the size of each letter, depending on the number of ascents preceding it in the sequence. Ascent sequences have recently been related to (2+2)-free posets and various other combinatorial structures. We study pattern avoidance in ascent sequences, giving several results for patterns of lengths up to 4, for Wilf equivalence and for growth rates. We establish bijective connections between pattern avoiding ascent sequences and various other combinatorial objects, in particular with set partitions. We also make a number of conjectures related to all of these aspects.
p-Ascent Sequences  [PDF]
Sergey Kitaev,Jeffrey Remmel
Mathematics , 2015,
Abstract: A sequence $(a_1, \ldots, a_n)$ of nonnegative integers is an {\em ascent sequence} if $a_0 =0$ and for all $i \geq 2$, $a_i$ is at most 1 plus the number of ascents in $(a_1, \ldots, a_{i-1})$. Ascent sequences were introduced by Bousquet-M\'elou, Claesson, Dukes, and Kitaev, who showed that these sequences of length $n$ are in 1-to-1 correspondence with \tpt-free posets of size $n$, which, in turn, are in 1-to-1 correspondence with interval orders of size $n$. Ascent sequences are also in bijection with several other classes of combinatorial objects including the set of upper triangular matrices with nonnegative integer entries such that no row or column contains all zeros, permutations that avoid a certain mesh pattern, and the set of Stoimenow matchings. In this paper, we introduce a generalization of ascent sequences, which we call {\em $p$-ascent sequences}, where $p \geq 1$. A sequence $(a_1, \ldots, a_n)$ of nonnegative integers is a $p$-ascent sequence if $a_0 =0$ and for all $i \geq 2$, $a_i$ is at most $p$ plus the number of ascents in $(a_1, \ldots, a_{i-1})$. Thus, in our terminology, ascent sequences are 1-ascent sequences. We generalize a result of the authors by enumerating $p$-ascent sequences with respect to the number of $0$s. We also generalize a result of Dukes, Kitaev, Remmel, and Steingr\'{\i}msson by finding the generating function for the number of $p$-ascent sequences which have no consecutive repeated elements. Finally, we initiate the study of pattern-avoiding $p$-ascent sequences.
Ascent sequences avoiding pairs of patterns  [PDF]
Andrew M. Baxter,Lara K. Pudwell
Mathematics , 2014,
Abstract: Ascent sequences were introduced by Bousquet-Melou et al. in connection with (2+2)-avoiding posets and their pattern avoidance properties were first considered by Duncan and Steingrimsson. In this paper, we consider ascent sequences of length $n$ avoiding two patterns of length 3, and we determine an exact enumeration for 16 different pairs of patterns. Methods include simple recurrences, bijections to other combinatorial objects (including Dyck paths and pattern-avoiding permutations), and generating trees. We also provide an analogue of the Erdos-Szekeres Theorem to prove that any sufficiently long ascent sequence contains either many copies of the same number or a long increasing subsequence, with a precise bound.
The Ascent of the Abundant: How Mutational Networks Constrain Evolution  [PDF]
Matthew C. Cowperthwaite ,Evan P. Economo,William R. Harcombe,Eric L. Miller,Lauren Ancel Meyers
PLOS Computational Biology , 2008, DOI: 10.1371/journal.pcbi.1000110
Abstract: Evolution by natural selection is fundamentally shaped by the fitness landscapes in which it occurs. Yet fitness landscapes are vast and complex, and thus we know relatively little about the long-range constraints they impose on evolutionary dynamics. Here, we exhaustively survey the structural landscapes of RNA molecules of lengths 12 to 18 nucleotides, and develop a network model to describe the relationship between sequence and structure. We find that phenotype abundance—the number of genotypes producing a particular phenotype—varies in a predictable manner and critically influences evolutionary dynamics. A study of naturally occurring functional RNA molecules using a new structural statistic suggests that these molecules are biased toward abundant phenotypes. This supports an “ascent of the abundant” hypothesis, in which evolution yields abundant phenotypes even when they are not the most fit.
Proximal Stochastic Dual Coordinate Ascent  [PDF]
Shai Shalev-Shwartz,Tong Zhang
Mathematics , 2012,
Abstract: We introduce a proximal version of dual coordinate ascent method. We demonstrate how the derived algorithmic framework can be used for numerous regularized loss minimization problems, including $\ell_1$ regularization and structured output SVM. The convergence rates we obtain match, and sometimes improve, state-of-the-art results.
A review of the ascent and emplacement of granitoid bodies into the crust
Katarína Bónová
Acta Montanistica Slovaca , 2005,
Abstract: This paper relates to basic information (i.e. mechanical aspects of ascent, indicators faciliting the discriminability of various ascent styles) about the models of ascent and emplacement of granitoid bodies, since the purely mechanical aspect of intrusion of magmas is a fascinating subject and it has generated a considerable controversy over many years. Individual models are demonstrated by world-known occurrences and examples from Western Carpathian s region. The conditions of magma migration are demonstrated as well.
Restricted ascent sequences and Catalan numbers  [PDF]
David Callan,Toufik Mansour,Mark Shattuck
Mathematics , 2014,
Abstract: Ascent sequences are those consisting of non-negative integers in which the size of each letter is restricted by the number of ascents preceding it and have been shown to be equinumerous with the (2+2)-free posets of the same size. Furthermore, connections to a variety of other combinatorial structures, including set partitions, permutations, and certain integer matrices, have been made. In this paper, we identify all members of the (4,4)-Wilf equivalence class for ascent sequences corresponding to the Catalan number C_n=\frac{1}{n+1}\binom{2n}{n}. This extends recent work concerning avoidance of a single pattern and provides apparently new combinatorial interpretations for C_n. In several cases, the subset of the class consisting of those members having exactly m ascents is given by the Narayana number N_{n,m+1}=\frac{1}{n}\binom{n}{m+1}\binom{n}{m}.
On 021-Avoiding Ascent Sequences  [PDF]
William Y. C. Chen,Alvin Y. L. Dai,Theodore Dokos,Tim Dwyer,Bruce E. Sagan
Mathematics , 2012,
Abstract: Ascent sequences were introduced by Bousquet-M\'{e}lou, Claesson, Dukes and Kitaev in their study of $(\bf{2+2})$-free posets. An ascent sequence of length $n$ is a nonnegative integer sequence $x=x_{1}x_{2}... x_{n}$ such that $x_{1}=0$ and $x_{i}\leq \asc(x_{1}x_{2}...x_{i-1})+1$ for all $1
Ascent sequences and the binomial convolution of Catalan numbers  [PDF]
Lara K. Pudwell
Mathematics , 2014,
Abstract: In this paper, we consider two sets of pattern-avoiding ascent sequences: those avoiding both 201 and 210 and those avoiding 0021. In each case we show that the number of such ascent sequences is given by the binomial convolution of the Catalan numbers. The result for $\{201, 210\}$-avoiders completes a family of results given by Baxter and the current author in a previous paper. The result for 0021-avoiders, together with previous work of Duncan, Steingr\'{i}msson, Mansour, and Shattuck, completes the Wilf classification of single patterns of length 4 for ascent sequences.
An Iterative Minimization Formulation for Saddle-Point Search  [PDF]
Weiguo Gao,Jing Leng,Xiang Zhou
Mathematics , 2014,
Abstract: This paper proposes and analyzes an iterative minimization formulation for search- ing index-1 saddle points of an energy function. This formulation differs from other eigenvector-following methods by constructing a new objective function near the guess at each iteration step. This leads to a quadratic convergence rate, in comparison to the linear case of the gentlest ascent dynamics (E and Zhou, nonlinearity, vol 24, p1831, 2011) and many other existing methods. We also propose the generalization of the new methodology for saddle points of higher index and for constrained energy functions on manifold.
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