String Problem Research Articles

Design of Multifunctional and Efficient Water-Based Annulus Protection Fluid for HTHP Sour Gas Wells

In order to solve the corrosion problem of production string in the process of acidizing for the purpose of production, a new water-based annular protective fluid suitable for HTHP acid gas, including H2S-CO2 wells, was developed. Firstly, an appropriate deoxidizer, bactericide, and corrosion inhibitor shall be selected according to the production string of acid gas. In addition, the synergism between additives is evaluated. Then, by designing the additive ratio, the optimal formulation of the water-based annular protective fluid is determined. Finally, a high-temperature autoclave was used to evaluate the protective performance of the water-based annular protective liquid. The results showed that it is recommended to use water-based annular protective liquids prepared with clear water that comes easily from nature (rivers, etc.), which consist of a corrosion inhibitor, CT2-19C (30,000 ppm), BN-45 bactericide (2 g/L), and anhydrous sodium sulfite (3 g/L). The density of the water-based annulus protection liquid is 1.02 g/cm3, and the freezing point is −2.01 °C. The dissolved oxygen content of water-based annulus protection fluids prepared with clear water in formation water shall be controlled within 0.3 ppm. The corrosion inhibition rate of water-based annular protective fluid in the liquid phase is higher than 90%, and the corrosion rate of P110SS steel in the gas–liquid phase is lower than the oilfield corrosion control index (0.076 mm/y).

Acceptability, Safety and Compliance of Copper T 380A as a Postpartum Intrauterine Contraceptive Device Inserted via Vaginal or Intracaesarean Route: A Cohort Study

Introduction: Considering the high unmet need for family planning in India, especially in postpartum mothers, there is a necessity for reliable, long-term, highly efficacious, safe and reversible contraception like Intrauterine Device (IUD) in these women. Copper T 380A (CuT 380A) as a Postpartum Intrauterine Contraceptive Device (PPIUCD) can reduce maternal and perinatal mortality by decreasing unintended and closely spaced pregnancies in the postpartum period. Despite many advantages, the acceptability and utilisation of PPIUCD still remain very low due to lack of information about it. Aim: To determine the acceptance, safety, and efficacy of Copper T 380A; its association with socio-demographic factors and to compare the clinical outcomes between vaginal and intracaesarean insertions of this device. Materials and Methods: A hospital-based prospective cohort study was conducted in the Department of Obstetrics and Gynaecology, RG Kar Medical College and Hospital, Kolkata, India, from February 2021 to July 2021 among the cohort of 210 consecutive eligible postpartum mothers delivered either vaginally or by caesarean section. Primarily, acceptance rate was assessed. Postinsertion follow-up was done till six months after delivery to note various clinical outcomes and complications measuring safety of the device in terms of cramping pain abdomen, irregular vaginal bleeding, abnormal vaginal discharge, string problems, and perforation; compliance in terms of pregnancy and expulsion; and continuation of the device. All the follow-up results were compared between two modes of insertions. Comparison between categorical variables was done by Chi-square test. Results: Out of 210 counselled mothers, Copper T 380A insertions were done in 178 (84.8%) postpartum mothers. The acceptance rate was slightly higher in intracaesarean group (n=87, 85.3%). The mean±Standard Deviation (SD) age of acceptance of IUCD in partcipants was 25.59±4.06 years with the highest rate observed in the age group of 21-25 years (n=82, 46.1%). Acceptance of the device was significantly associated with multipara and the clients completed primary education (p-value <0.05). On follow-up, overall complications are low. No case of perforation was reported. Significantly higher incidences of abnormal vaginal bleeding, foul smelling vaginal discharge, string problems, and expulsions were observed in the vaginal group (p-value <0.05). Pregnancy was recorded in five (5.5%) clients and all of them were in the vaginal group. Out of 178 acceptors, compliance-continuation of the device was shown by 151 (84.8%), with significantly higher in the caesarean group (p-value <0.000051). A significantly higher continuation rate was observed among the clients of low socio-economic status (p-value <0.000207). Abdominal pain (33.33%) was reported to be the most common reason for discontinuation of Copper T 380A. Conclusion: Regardless of the mode of delivery, Copper T 380A is quite an acceptable, safe, effective, and convenient contraceptive method with a good continuation rate in the immediate postpartum period.

Quantum Meets Fine-Grained Complexity: Sublinear Time Quantum Algorithms for String Problems

Longest common substring (LCS), longest palindrome substring (LPS), and Ulam distance (UL) are three fundamental string problems that can be classically solved in near linear time. In this work, we present sublinear time quantum algorithms for these problems along with quantum lower bounds. Our results shed light on a very surprising fact: Although the classic solutions for LCS and LPS are almost identical (via suffix trees), their quantum computational complexities are different. While we give an exact {{tilde{O}}}(sqrt{n}) time algorithm for LPS, we prove that LCS needs at least time tilde{Omega }(n^{2/3}) even for 0/1 strings.

Control of drill string torsional vibrations using optimal static output feedback

Failures during oil extraction can have impacts on exploration costs and time, especially those caused by vibrations during the drilling phase. A proper way to reduce these detrimental failures is to design an active controller that efficiently mitigates vibrations with the constraint of using the limited amount of real time data available in field operations. In this context, a technique that might be suitable for the controller design is the optimal static output feedback (OSOF), that considers only the measured signals for feedback. Recently, the authors proposed some modifications to this technique, involving the addition of sensors locations and a new way of dealing with the dependence of the controller parameters on the system initial condition. The aim of this contribution is to apply this novel control technique to suppress drill string torsional vibrations and address all the particular aspects regarding the application of the proposed controller for the drill string problem, i.e., reformulation of the equations of motion as a stabilization problem, determination of the output matrix and linearization. Analyses and comparison of the proposed controller with proportional–integral (PI) and linear quadratic regulator (LQR) controllers are performed considering numerical simulations of a finite element model with a non-regularized dry friction function. Results indicate that the proposed controller has a performance similar to the LQR with a significantly reduced number of sensors, and also exhibits superior performance and robustness with an equivalent control effort over the PI controller.

A Novel Approach in Solving Improper Integrals

To resolve several challenging applications in many scientific domains, general formulas of improper integrals are provided and established for use in this article. The suggested theorems can be considered generators for new improper integrals with precise solutions, without requiring complex computations. New criteria for handling improper integrals are illustrated in tables to simplify the usage and the applications of the obtained outcomes. The results of this research are compared with those obtained by I.S. Gradshteyn and I.M. Ryzhik in the classical table of integrations. Some well-known theorems on improper integrals are considered to be simple cases in the context of our work. Some applications related to finding Green’s function, one-dimensional vibrating string problems, wave motion in elastic solids, and computing Fourier transforms are presented.

Limitations of Mean-Based Algorithms for Trace Reconstruction at Small Edit Distance

Trace reconstruction considers the task of recovering an unknown string <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {x}\in \{0,1\}^{n}$ </tex-math></inline-formula> given a number of independent “traces”, i.e., subsequences of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {x}$ </tex-math></inline-formula> obtained by randomly and independently deleting every symbol of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {x}$ </tex-math></inline-formula> with some probability <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p$ </tex-math></inline-formula> . The information-theoretic limit of the number of traces needed to recover a string of length <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> is still unknown. This limit is essentially the same as the number of traces needed to determine, given strings <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {x}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {y}$ </tex-math></inline-formula> and traces of one of them, which string is the source. The most-studied class of algorithms for the worst-case version of the problem are “mean-based” algorithms. These are a restricted class of distinguishers that only use the mean value of each coordinate on the given samples. In this work we study limitations of mean-based algorithms on strings at small Hamming or edit distance. We show that, on the one hand, distinguishing strings that are nearby in Hamming distance is “easy” for such distinguishers. On the other hand, we show that distinguishing strings that are nearby in edit distance is “hard” for mean-based algorithms. Along the way, we also describe a connection to the famous Prouhet-Tarry-Escott (PTE) problem, which shows a barrier to finding explicit hard-to-distinguish strings: namely such strings would imply explicit short solutions to the PTE problem, a well-known difficult problem in number theory. Furthermore, we show that the converse is also true, thus, finding explicit solutions to the PTE problem is equivalent to the problem of finding explicit strings that are hard-to-distinguish by mean-based algorithms. Our techniques rely on complex analysis arguments that involve careful trigonometric estimates, and algebraic techniques that include applications of Descartes’ rule of signs for polynomials over the reals.

Inverse Nodal Problem for p-Laplacian String Equation with Prüfer Substitution

We consider an inverse nodal problem for p-Laplacian string equation under some boundary conditions. Asymptotic formulas for eigenvalues and nodal parameters are constructed by modified Pr¨ufer substitution. The most important process is to apply modified Pr¨ufer substitution to get an exhaustive asymptotic estimate for eigenvalues. Moreover, a reconstruction formula for density function of p-aplacian string equation is obtained by nodal parameters. Generated outcomes are the generalization of the known string problem.

A 2‐component Camassa‐Holm Equation, Euler‐Bernoulli Beam Problem, and Noncommutative Continued Fractions

AbstractA new approach to the Euler‐Bernoulli beam based on an inhomogeneous matrix string problem is presented. Three ramifications of the approach are developed: motivated by an analogy with the Camassa‐Holm equation a class of isospectral deformations of the beam problem is formulated; a reformulation of the matrix string problem in terms of a certain compact operator is used to obtain basic spectral properties of the inhomogeneous matrix string problem with Dirichlet boundary conditions; the inverse problem is solved for the special case of a discrete Euler‐Bernoulli beam. The solution involves a noncommutative generalization of Stieltjes’ continued fractions, leading to the inverse formulas expressed in terms of ratios of Hankel‐like determinants. © 2022 Courant Institute of Mathematics and Wiley Periodicals LLC.

Solving String Problems on Graphs Using the Labeled Direct Product

Suffix trees are an important data structure at the core of optimal solutions to many fundamental string problems, such as exact pattern matching, longest common substring, matching statistics, and longest repeated substring. Recent lines of research focused on extending some of these problems to vertex-labeled graphs, either by using efficient ad-hoc approaches which do not generalize to all input graphs, or by indexing difficult graphs and having worst-case exponential complexities. In the absence of an ubiquitous and polynomial tool like the suffix tree for labeled graphs, we introduce the labeled direct product of two graphs as a general tool for obtaining optimal algorithms in the worst case: we obtain conceptually simpler algorithms for the quadratic problems of string matching (SMLG) and longest common substring (LCSP) in labeled graphs. Our algorithms run in time linear in the size of the labeled product graph, which may be smaller than quadratic for some inputs, and their run-time is predictable, because the size of the labeled direct product graph can be precomputed efficiently. We also solve LCSP on graphs containing cycles, which was left as an open problem by Shimohira et al. in 2011. To show the power of the labeled product graph, we also apply it to solve the matching statistics (MSP) and the longest repeated string (LRSP) problems in labeled graphs. Moreover, we show that our (worst-case quadratic) algorithms are also optimal, conditioned on the Orthogonal Vectors Hypothesis. Finally, we complete the complexity picture around LRSP by studying it on undirected graphs.

Weighted string equation where the weight is a noncompact multiplier: continuous spectrum and eigenvalues

The oscillation equation for a singular string with discrete weight generated by a self-similar n n -link multiplier in the Sobolev space with a negative smoothness index is considered. It is shown that in the case of a noncompact multiplier, the string problem is equivalent to the spectral problem for an ( n − 1 ) (n-1) -periodic Jacobi matrix. In the case of n = 3 n=3 , a complete description of the spectrum of the problem is given, and a criterion for emergence of an eigenvalue in a gap of the continuous spectrum is obtained.

On the role of metaheuristic optimization in bioinformatics

AbstractMetaheuristic algorithms are employed to solve complex and large‐scale optimization problems in many different fields, from transportation and smart cities to finance. This paper discusses how metaheuristic algorithms are being applied to solve different optimization problems in the area of bioinformatics. While the text provides references to many optimization problems in the area, it focuses on those that have attracted more interest from the optimization community. Among the problems analyzed, the paper discusses in more detail the molecular docking problem, the protein structure prediction, phylogenetic inference, and different string problems. In addition, references to other relevant optimization problems are also given, including those related to medical imaging or gene selection for classification. From the previous analysis, the paper generates insights on research opportunities for the Operations Research and Computer Science communities in the field of bioinformatics.

The Moral Resources of Constitutions in Africa and the Three Problems to Interpretation?

This paper attempts to conceive the constitutions of African states in the post-colonial context, especially Ghana and Nigeria, as laws in a manner that will reflect the moral resources necessary to protect the wellbeing of their people. This is expected to be done through judicial interpretation of the constitutions. But this project might be frustrated by three problems. The problems of the multicultural strings; the colonial and post-colonial dualism; and the transitional democratic premises of these states. Notwithstanding these problems, I argue that the best value by which constitutions in Ghana and Nigeria can be assigned through judicial interpretation is adherent to a particular theory of law that priorities reason over the positivist character of the constitutions. Keywords : Constitution, Positivism, Constitutional Interpretation, Multicultural Strings, Post-Colonial Dualism, Transitional Democratic Premise, Moral Resources of Constitution, Fundamental Law, Human Rights, Rule of Law, Constitutional Positivism. DOI: 10.7176/JLPG/120-16 Publication date: April 30 th 2022

Hermite–Padé approximations with Pfaffian structures: Novikov peakon equation and integrable lattices

Motivated by the Novikov equation and its peakon problem, we propose a new mixed type Hermite–Padé approximation whose unique solution is a sequence of polynomials constructed with the help of Pfaffians. These polynomials belong to the family of recently proposed partial-skew-orthogonal polynomials. The relevance of partial-skew-orthogonal polynomials is especially visible in the approximation problem germane to the Novikov peakon problem so that we obtain explicit inverse formulae in terms of Pfaffians by reformulating the inverse spectral problem for the Novikov multipeakons. Furthermore, we investigate two Hermite–Padé approximations for the related spectral problem of the discrete dual cubic string, and show that these approximation problems can also be solved in terms of partial-skew-orthogonal polynomials and nonsymmetric Cauchy biorthogonal polynomials. This formulation results in a new correspondence among several integrable lattices.

Quantum Algorithms for Some Strings Problems Based on Quantum String Comparator

We study algorithms for solving three problems on strings. These are sorting of n strings of length k, “the Most Frequent String Search Problem”, and “searching intersection of two sequences of strings”. We construct quantum algorithms that are faster than classical (randomized or deterministic) counterparts for each of these problems. The quantum algorithms are based on the quantum procedure for comparing two strings of length k in O(k) queries. The first problem is sorting n strings of length k. We show that classical complexity of the problem is Θ(nk) for constant size alphabet, but our quantum algorithm has O˜(nk) complexity. The second one is searching the most frequent string among n strings of length k. We show that the classical complexity of the problem is Θ(nk), but our quantum algorithm has O˜(nk) complexity. The third problem is searching for an intersection of two sequences of strings. All strings have the same length k. The size of the first set is n, and the size of the second set is m. We show that the classical complexity of the problem is Θ((n+m)k), but our quantum algorithm has O˜((n+m)k) complexity.

Population Diversity in Ant-inspired Optimization Algorithms

Finding a balance between exploration and exploitation is very important in the case of metaheuristics optimization, especially in the systems leveraging population of individuals expressing (as in Evolutionary Algorithms, etc.) or constructing (as in Ant Colony Optimization) solutions. Premature convergence is a real problem and finding means of its automatic detection and counteracting are of great importance. Measuring diversity in Evolutionary Algorithms working in real-value search space is often computationally complex, but feasible while measuring diversity in combinatorial domain is practically impossible (cf. Closest String Problem). Nevertheless, we propose several practical and feasible diversity measurement techniques dedicated to Ant Colony Optimization algorithms, leveraging the fact that even though analysis of the search space is at least an NP problem, we can focus on the pheromone table, where the direct outcomes of the search are expressed and can be analyzed. Besides proposing the measurement techniques, we apply them to assess the diversity of several variants of ACO, and closely analyze their features for the classic ACO. The discussion of the results is the first step towards applying the proposed measurement techniques in auto-adaptation of the parameters affecting directly the exploitation and exploration features in ACO in the future.

Using the longest run subsequence problem within homology-based scaffolding

Genome assembly is one of the most important problems in computational genomics. Here, we suggest addressing an issue that arises in homology-based scaffolding, that is, when linking and ordering contigs to obtain larger pseudo-chromosomes by means of a second incomplete assembly of a related species. The idea is to use alignments of binned regions in one contig to find the most homologous contig in the other assembly. We show that ordering the contigs of the other assembly can be expressed by a new string problem, the longest run subsequence problem (LRS). We show that LRS is NP-hard and present reduction rules and two algorithmic approaches that, together, are able to solve large instances of LRS to provable optimality. All data used in the experiments as well as our source code are freely available. We demonstrate its usefulness within an existing larger scaffolding approach by solving realistic instances resulting from partial Arabidopsis thaliana assemblies in short computation time.

LP-based heuristics for the distinguishing string and substring selection problems

LP-based heuristics for the distinguishing string and substring selection problems

Experimental research on the dynamic buckling of a tubular string when hitting the bottom of a vertical well

The buckling problem of the tubular string in a wellbore has historically been the focus of petroleum engineering research. Most scholars have studied the static buckling problem of the tubular string by simplifying the model. However, the string occasionally breaks off or slips during drilling and production operations, which causes the string to slip along the wellbore and eventually hits the bottom of the well. Under this condition, the string is subjected to transient impact loads, which may lead to dynamic sinusoidal buckling, helical buckling, and even plastic deformation or fracture. At present, no one has studied the phenomenon when the tubular string hits the bottom of the well, this article is the first time to study this phenomenon. A simulation experiment platform for the tubular string hitting the bottom of a vertical well is built. Experiments were carried out to study this phenomenon of buckling of the string of varying sizes striking the bottom of a vertical well at different impact speeds using the controlled variable method. In the process of the tubular string hitting the bottom, the tubular string undergoes no buckling, plane buckling, helical buckling combined rotating, and rebounding combined reverse rotating. The experimental conclusions provide crucial guidance for theoretical research and simulation.

Fixed-Parameter Tractability of Crossover: Steady-State GAs on the Closest String Problem

We investigate the effect of crossover in the context of parameterized complexity on a well-known fixed-parameter tractable combinatorial optimization problem known as the closest string problem. We prove that a multi-start ( $$\mu$$ +1) GA solves arbitrary length-n instances of closest string in $$2^{O(d^2 + d \log k)} \cdot t(n)$$ steps in expectation. Here, k is the number of strings in the input set, d is the value of the optimal solution, and $$n \le t(n) \le {\text {poly}}(n)$$ is the number of iterations allocated to the ( $$\mu$$ +1) GA before a restart, which can be an arbitrary polynomial in n. This confirms that the multi-start ( $$\mu$$ +1) GA runs in randomized fixed-parameter tractable (FPT) time with respect to the above parameterization. On the other hand, if the crossover operation is disabled, we show there exist instances that require $$n^{\varOmega (\log (d+k))}$$ steps in expectation. The lower bound asserts that crossover is a necessary component in the FPT running time.

On the issue of corrosion in the operation of oil and gas wells

The problem of leakage production strings in production and injection wells is becoming more and more urgent every year in Russia’s fields. As a result, the water encroachment of produced products increases, and as a result, the operation of wells becomes unprofitable. The main reasons for the leakage of wells are aging of well stock, poor quality cementation during final works during drilling, corrosion processes in places where aquifers come into contact with the string, killing of wells at pressures higher than the pressure of pressing, depressurization in coupling joints, metal corrosion and other technological reasons. Identification of factors affecting the deformation of production strings in the process of well corrosion is necessary and important, as it is taken into account in the development of measures to increase the accident-free operation of wells. 725 wells of PJSC TATNEFT were analyzed in this work. The analysis revealed that one of the reasons for the corrosion of production strings is the presence of lost circulation horizons, which confirms the analysis.