Bubble Space Research Articles

Portfolio optimization in the capital market bubble space, an application of meta-heuristic algorithms

PurposeIn this study, to maximize returns and minimize investment risk, an attempt was made to form an optimal portfolio under conditions where the capital market has a price bubble. According to the purpose, the research was of the applied type, in terms of data, quantitative and postevent, and in terms of the type of analysis, it was of the descriptive-correlation type. Sequence, skewness and kurtosis tests were used to identify the months with bubbles from 2015 to 2021 in the Tehran Stock Exchange. After identifying the bubble courses, artificial bee colony meta-heuristic and invasive weed algorithms were used to optimize the portfolio. The purpose of this paper is to address these issues.Design/methodology/approachThe existence of bubbles in the market, especially in the capital market, can prevent the participation of investors in the capital market process and the correct allocation of financial resources for the economic development of the country. However, due to the goal of investors to achieve a portfolio of high returns with the least amount of risk, there is need to pay attention to these markets increases.FindingsThe results identify 14 periods of price bubbles during the study period. Additionally, stock portfolios with maximum returns and minimum risk were selected for portfolio optimization. According to the results of using meta-heuristic algorithms to optimize the portfolio, in relation to the obtained returns and risk, no significant difference was observed between the returns and risk of periods with price bubbles in each of the two meta-heuristic algorithms. This study can guide investors in identifying bubble courses and forming an optimal portfolio under these conditions.Research limitations/implicationsOne of the limitations of this research is the non-generalizability of the findings to stock exchanges of other countries and other time periods due to the condition of the price bubble, as well as other companies in the stock market due to the restrictions considered for selecting the statistical sample.Originality/valueThis study intends to form an optimal stock portfolio in a situation where the capital market suffers from a price bubble. This study provides an effective and practical solution for investors in the field of stock portfolio optimization.

A family of tree-based generators for bubbles in directed graphs

Bubbles are pairs of internally vertex-disjoint $(s, t)$-paths in a directed graph. In de Bruijn graphs built from reads of RNA and DNA data, bubbles represent interesting biological events, such as alternative splicing (AS) and allelic differences (SNPs and indels). However, the set of all bubbles in a de Bruijn graph built from real data is usually too large to be efficiently enumerated and analysed in practice. In particular, despite significant research done in this area, listing bubbles still remains the main bottleneck for tools that detect AS events in a reference-free context. Recently, in [Acuña et al., Algorithmica 2020] the concept of a bubble generator was introduced as a way for obtaining a compact representation of the bubble space of a graph. Although this bubble generator was quite effective in finding AS events, preliminary experiments showed that it is about 5 times slower than state-of-art methods. In this paper we propose a new family of bubble generators which improve substantially on previous work: bubble generators in this new family are about two orders of magnitude faster and are still able to achieve similar precision in identifying AS events. To highlight the practical value of our new bubble generators, we also report some experimental results on real datasets. Keywords: bubble generator, directed graphs, alternative splicing

Characteristic analysis of concrete air entraining agents in different media

The effects of six concrete air-entraining agents (AEAs) in different media, including saturated calcium hydroxide solution, cement paste, mortar, fresh concrete, and hardened concrete (subjected to different vibration treatments), were compared. The results show that the lower the AEA concentration is at the surface tension inflection point of a saturated calcium hydroxide solution, the less AEA is needed to obtain the same air content in the concrete. Similarly, AEAs with better air-entraining performance lead to a higher initial cement paste foam height and a lower mortar density. The differences between the air content of fresh concrete and hardened concrete are large; high-frequency vibration can significantly optimize the bubble space of low-slump concrete.

Maximum entropy-based variational multiscale element-free Galerkin methods for scalar advection–diffusion problems

This paper presents a new parameter-free stability term via Galerkin mesh-free scheme to solve the multiscale behavior of steady-state advection-dominated advection–diffusion problems where the classical Galerkin method is shown to be unstable. A variational multiscale element-free Galerkin method (VMEFG) is proposed based on first-order local maximum entropy (LME) basis functions, which are owning weak Kronecker delta property that enables the imposition of essential boundary conditions with ease. In the present work, the finite space of the problem is enlarged by the infinite bubble space in the background integration cell. These bubbles constitute the fine scale solve locally (captures) the thin layers associated with the flow. Then, this bubble space solution is substituted into the coarse scales to recover the VMEFG method globally, which determines the parameter-free stability term naturally. The developed LME-based VMEFG is studied against three standard benchmark problems exhibiting boundary/internal layers and observed to be stable and robust in resolving all the layers. The effect of various values of locality parameter in defining the LME approximants is carried out between the values 1.2 and 2. It is found that the suboptimal solution is arrived at the locality parameter value, 1.5 and above 1.8, is not recommended. Convergence test is also carried out with two different mass functions and validated with the analytical solution and found that the computational efficiency of the present method is in good agreement with analytical solution.

On Bubble Generators in Directed Graphs

Bubbles are pairs of internally vertex-disjoint (s, t)-paths in a directed graph, which have many applications in the processing of DNA and RNA data. Listing and analysing all bubbles in a given graph is usually unfeasible in practice, due to the exponential number of bubbles present in real data graphs. In this paper, we propose a notion of bubble generator set, i.e., a polynomial-sized subset of bubbles from which all the other bubbles can be obtained through a suitable application of a specific symmetric difference operator. This set provides a compact representation of the bubble space of a graph. A bubble generator can be useful in practice, since some pertinent information about all the bubbles can be more conveniently extracted from this compact set. We provide a polynomial-time algorithm to decompose any bubble of a graph into the bubbles of such a generator in a tree-like fashion. Finally, we present two applications of the bubble generator on a real RNA-seq dataset.

AC0-nonconforming quadrilateral finite element for the fourth-order elliptic singular perturbation problem

In this paper, aC0nonconforming quadrilateral element is proposed to solve the fourth-order elliptic singular perturbation problem. For each convex quadrilateralQ, the shape function space is the union ofS21(Q*) and a bubble space. The degrees of freedom are defined by the values at vertices and midpoints on the edges, and the mean values of integrals of normal derivatives over edges. The local basis functions of our element can be expressed explicitly by a new reference quadrilateral rather than by solving a linear system. It is shown that the method converges uniformly in the perturbation parameter. Lastly, numerical tests verify the convergence analysis.

Higher dimensional formal loop spaces

If $M$ is a symplectic manifold then the space of smooth loops $\mathrm C^{\infty}(\mathrm S^1,M)$ inherits of a quasi-symplectic form. We will focus in this article on an algebraic analogue of that result. In 2004, Kapranov and Vasserot introduced and studied the formal loop space of a scheme $X$. We generalize their construction to higher dimensional loops. To any scheme $X$ -- not necessarily smooth -- we associate $\mathcal L^d(X)$, the space of loops of dimension $d$. We prove it has a structure of (derived) Tate scheme -- ie its tangent is a Tate module: it is infinite dimensional but behaves nicely enough regarding duality. We also define the bubble space $\mathcal B^d(X)$, a variation of the loop space. We prove that $\mathcal B^d(X)$ is endowed with a natural symplectic form as soon as $X$ has one (in the sense of [PTVV]). Throughout this paper, we will use the tools of $(\infty,1)$-categories and symplectic derived algebraic geometry.

Corrosion casts of big bubbles formed during deep anterior lamellar keratoplasty.

To characterize the walls of big bubbles formed during deep anterior lamellar keratoplasty (DALK) using the corrosion casting technique. Fresh corneoscleral buttons with normal transparency and without any known eye diseases (n=11) were obtained from 11 human donors. A 20-gauge needle was used to inject a solution of 20% polyvinyl alcohol (PVA) immediately beneath the corneal endothelium to form big bubbles in eight corneoscleral buttons. In the second experiment on three corneoscleral buttons, a big bubble was first formed by air injection beneath the endothelium. Thereafter, 20% PVA was injected into the bubble space. Scanning electron microscopy was used to characterize the surfaces of the casts, which replicated the walls of the big bubbles. A type-1 bubble was formed in all corneas. In one cornea, one type-1 bubble was initially formed centrally, and while it was enlarged, an eccentric type-2 bubble appeared. Scanning electron microscopy showed that the casts of type-1 bubbles had two distinct surfaces. The anterior surface demonstrated several holes or pits, depending on the material used for the bubble formation, whereas the posterior surface exhibited an uneven surface. The anterior and posterior surfaces of the type-2 cast were more or less similar. A communication measuring 531.9µm in length and 171.4µm in diameter was found between the two bubbles. The corrosion casting technique provides a permanent three-dimensional record of the potential spaces and barriers in the posterior corneal stroma, which explains several features associated with big-bubble DALK.

RGB-D based place representation in topological maps

With the recent developments in sensor technology including Microsoft Kinect, it has now become much easier to augment visual data with three-dimensional depth information. In this paper, we propose a new approach to RGB-D based topological place representation--building on bubble space. While bubble space representation is in principle transparent to the type and number of sensory inputs employed, practically, this has been only verified with visual data that are acquired either via a two degrees of freedom camera head or an omnidirectional camera. The primary contribution of this paper is of practical nature in this perspective. We show that bubble space representation can easily be used to combine RGB and depth data while affording acceptable recognition performance even with limited field of view sensing and simple features.

Freeze–thaw resistance of alkali–slag concrete based on response surface methodology

Alkali–slag concrete (ASC), with the frost resistant grade of above F300 and frost resistant coefficient DF of about 90%, is prepared using slag and composite activator composed of Na2SiO3 and NaOH. Response surface methodology (RSM) is applied to study the freeze–thaw resistance of ASC. The effects of activator solution–slag ratio (A/S), slag content and sand ratio on the freeze–thaw resistance are analyzed using the softwares of Design Expert and Box-Benhnken Design (BBD). Models are established for DF and the influence of air-void structure of hard concrete on the freeze–thaw resistance, respectively. The result shows that the DF model coincides well with the test results and can be used to analyze and predict the freeze–thaw resistance of ASC. The influence on the freeze–thaw resistance from high to low is A/S, slag content and sand ratio. The interaction of A/S and slag content is the most prominent and air-void structure is the crucial factor. The air bubble space coefficient and its specific surface area have good correlation with DF. The freeze–thaw resistance tends to better with smaller air bubble space coefficient and bigger specific surface area.

Bubble space and place representation in topological maps

This paper presents bubble space based representation of “places” (nodes) in topological maps. Bubble space simultaneously provides for detailed (bubble surfaces) and holistic (bubble descriptors) representation of places. It is based on bubble memory where visual feature values and their local S2-metric relations from robot’s viewpoint are simultaneously encoded on a deformable spherical surface. Bubble surfaces extend bubble memory to accommodate varying robot pose and multiple features. They are transformed into bubble descriptors that are rotationally invariant with respect to heading changes while being computable in an incremental manner as each new set of visual observations is made. We use bubble descriptors for place learning and recognition with support vector machines in both indoor and outdoor environments and provide analysis results on recognition, recall and precision rates and time performance including a comparative study with the state-of-the-art descriptors.

Energy absorption of foamed concrete from low-velocity impacts

Foamed concrete has existing uses as an energy absorber for large-scale and low-cost applications, such as explosion prevention in mines and military engineering projects and aircraft arresting systems. However, there is little information enumerating the energy absorption characteristics of different foamed concretes; this paper describes a study that determined these using low-velocity impact tests. A range of foamed concretes were tested with plastic densities varying from 500 to 1400 kg/m3, which are typical of those produced commercially. The tests used hemispherical projectiles with a range of masses from 1·2 to 8·4 kg, dropped from a fixed height of 4·7 m. The resulting ‘damage' was measured and related to the impact energy imparted to the test specimen. A classification, using five different failure modes, was established and the mode in which the test specimen neither caused ‘rebound' of the projectile nor itself fractured was selected as representing ‘ideal' energy absorption behaviour. Making the assumption of no energy losses and using the conservation of energy law, the energy absorption capacity of foamed concrete was calculated and was found to vary from 4 to 15 MJ/m3 for different mixes. The optimum energy absorption was noted for the 1000 kg/m3 mix at water to cement (w/c) ratios from 0·6 to 0·7, which allowed a debris field of crushed foamed concrete to form and be pushed into the concrete bubble space ahead of the nose of the projectile, without fracturing.

Ophthalmic viscosurgical device–assisted incision modification for the big-bubble technique in deep anterior lamellar keratoplasty

We describe several simple modifications of the big-bubble technique to improve surgeon comfort and reduce the risk for complications, with emphasis on our ophthalmic viscosurgical device (OVD)-assisted incision technique. By coating the overlying stroma with OVD prior to entering the big bubble, space is maintained in the pre-Descemet plane. This prevents collapse of the big bubble and allows an air-OVD exchange. We have successfully used this technique in 72 consecutive cases without a perforation during the entry incision to the bubble space of the DALK procedure. No author has a financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Efficient hydrogen generation from organic chemical hydrides by using catalytic reactor on the basis of superheated liquid-film concept

Under boiling and refluxing conditions for catalytic dehydrogenation of organic chemical hydrides (decalin, methylcyclohexane and others) in a batch-wise reactor, either suspended states with excess amounts of substrate or sand-bath states with its scarce amounts were found to be inferior generally to the so-called “liquid-film states” with adequate amount ratios of substrate to catalyst, where the catalyst-layer temperatures were superheated or raised higher than the boiling point, and, consequently, reactivities became more favorable at higher heating temperatures in contrast to the boiling suspended states. Equilibrium shifts due to reactive distillation were well demonstrated under boiling and refluxing conditions in naphthene dehydrogenation. Moreover, desorption of hydrogen from the active sites to the bubble space was enhanced in the superheated liquid-film states, with large translational entropy endowed. Provided the extents of equilibrium deviation were large enough (Prigogine's approach), thermodynamic couplings among irreversible processes would be realized between heat transfer and mass transfer as an example of the extended De Donder's equation. Restriction of chemical equilibrium could be removed under temperature gradient conditions, as the consequence that its Gibbs energy change became more negative than that under iso-temperature conditions. Within the framework of irreversible thermodynamics, the decreased retardation constant K in the superheated liquid-film states was interpreted in terms of a vector-level coupling between temperature gradient and desorption. Moreover, vigorous bubble formation would give additional favor to the reaction rates owing to enlarged repeating frequencies of sequential non-microreversible processes in dehydrogenation catalysis. Organic chemical hydrides are attractive from the viewpoints of safe, economical, exergy-saving and large hydrogen contents for hydrogen storage and distribution. Their main defects have been pointed out hitherto that the endothermic reaction temperatures are too high. In this paper, a new concept on superheated liquid-film catalysis is explored for dehydrogenation temperatures to decrease, which would result in not only saving exergy for external heating but also avoiding catalyst deactivation due to carbon deposit.

LINK-CUTTING BUBBLES FOR THE STABILIZATION OF CONVECTION-DIFFUSION-REACTION PROBLEMS

It is known that the addition and elimination of suitable bubble functions can result in a stabilized scheme of the SUPG-type. Residual-Free Bubbles (RFB), in particular, can assure a quasi-optimal stabilized scheme, but they are difficult to compute in one dimension and nearly impossible to compute in two and three dimensions, unless in special limit cases. Strongly convection-dominated problems (without reaction terms) are one of these cases, where it is possible to find reasonably simple computable bubbles that provide a stabilizing effect as good as that of true RFB. Here, although in a one-dimensional framework, we analyze the case in which a non-negligible reaction term is present, and provide a simple recipe for spotting a suitable bubble space (adding two bubbles to each element) that provides a very good stabilizing effect. The method adapts very well to all regimes with continuous transitions from one regime to another. It is clear that the one-dimensional case, in itself, has no real interest. We believe, however, that the discussion can cast some light on the interaction between convection and reaction that could be useful in future works dealing with multidimensional, more realistic problems.

New approaches to error estimation and adaptivity for the Stokes and Oseen equations

SUMMARY We present a new approach to deliver reliable approximations of the norm of the residuals resulting from finite element solutions to the Stokes and Oseen equations. The method is based upon a global solve in a bubble space using iterative techniques. This provides an alternative to the classical equilibrated element residual methods for which it is necessary to construct proper boundary conditions for each local problem. The method is first used to develop a global a posteriori error estimator. It is then applied in a strategy to control the numerical error in specific outputs or quantities of interest which are functions of the solutions to the Stokes and Oseen equations. Copyright © 1999 John Wiley & Sons, Ltd. In recent years, the goal in a posteriori error estimation for computational processes has drifted from evaluating the numerical error in the classical energy norm to estimating it in terms of quantities of practical interest. Works in this field have been undertaken in [1‐3] with the objective to estimate and:or control the error by adapting the mesh parameters with respect to these quantities. The methodology, developed in [4], is extended here to non-self-adjoint problems, specifically the Oseen equations. It is based on the computation of functions which relate the influence of the residuals, viewed as the sources of error in the finite element approximations, onto the error quantity of interest. However, this technique relies on accurate evaluation of the norm of the residuals and on global error estimators in the ‘energy’ norm. We then propose a method which belongs to the family of Implicit Error Residual methods, for which norms of the residuals Rh in the momentum equation and Rh in the continuity equation are post-processed to provide meaningful error estimates. The computation of the norm of Rh is shown to be exact and cheap. The calculation of the norm of Rh is however more demanding. A new technique is developed which provides accurate approximations of Rh in a space of bubble functions which is gradually enriched through a global but inexpensive iterative process.

New approaches to error estimation and adaptivity for the Stokes and Oseen equations

We present a new approach to deliver reliable approximations of the norm of the residuals resulting from finite element solutions to the Stokes and Oseen equations. The method is based upon a global solve in a bubble space using iterative techniques. This provides an alternative to the classical equilibrated element residual methods for which it is necessary to construct proper boundary conditions for each local problem. The method is first used to develop a global a posteriori error estimator. It is then applied in a strategy to control the numerical error in specific outputs or quantities of interest which are functions of the solutions to the Stokes and Oseen equations. Copyright © 1999 John Wiley & Sons, Ltd.

Enhancement of the total acoustic field due to the coupling effects from a rough sea surface and a bubble layer

At wind speeds higher than a few meters per second, when breaking waves are present, the sea surface roughness is accompanied by an assemblage of microbubbles forming different structures or clouds, varying in range and in time. For simplicity, in terms of an average bubble population, the time and space varying assemblage of microbubbles is often assumed to be uniform and referred to as the subsurface bubble layer. Of fundamental importance is the role that this subsurface bubble layer may play in connection with scattering from a rough air/sea interface. The purpose of this paper is to determine the effect that the rough surface in the presence of this subsurface bubble layer has on the total field at low frequencies. Numerical calculations indicate that at low frequencies the bubble layer shifts the incident angle on the surface to steeper angles. In addition it was found that the enhancement of the total field is a consequence of scattering at the rough surface in the presence of the upper refracting bubble layer. An enhancement of approximately 40 dB with respect to the bubble-free medium was obtained very near the surface for a frequency of 400 Hz, with a nominal grazing angle of 20 deg, and a void fraction at the surface of 3.2×10−5. The enhancement decreases to about 10–15 dB at 10 m below the surface, but is still significant at depth exceeding the bubbly region. For a void fraction at the surface of 3.1×10−6 the enhancement is approximately 5 dB.

A locking-free finite element method for Naghdi shells

In this paper a locking-free method, using mixed finite elements, is introduced to approximate the solution of Naghdi shell problems with small parameter t, the thickness of the shell. The approach of Arnold and Brezzi (1997) is employed with some important changes. Instead of proving the discrete Inf-Sup condition for arbitrary geometric coefficients, which does not seem possible, we prove a weaker stability condition for smooth enough geometrically dependent coefficients, which is sufficient for deducing the optimal error estimate as long as h 2 t is uniformly bounded. For extremely small t, we can relax this condition either using a larger bubble space or stabilizing the problem by replacing t 2 by t 2 + h 4. In either case an optimal error estimate still holds.

A large nuclear accretion disk in the active galaxy NGC4261

THE powerful emissions from the nuclei of active galaxies and quasars are thought to arise from the accretion of matter onto a massive black hole. Angular momentum will prevent matter from falling directly onto the central mass; instead, an 'accretion disk' should form, within which the gravitationally bound material will lose angular momentum and gradually spiral inwards. Accretion disks in active galactic nuclei have not hitherto been observed directly, but theoretical models1 suggest that they are comparable in size to the gravitational radius of the central black hole (about 1013 cm) and are heated to high temperatures by the energy released from the accreting matter. Using the Planetary Camera on the Bubble Space Telescope, we have discovered an unexpectedly large (2 x 1020 cm radius) disk of cool dust and gas surrounding a bright unresolved nucleus in theactive galaxy NGC4261. We suggest that the bright point corresponds to thermal emission from the hot disk favoured by theoreticians, and that this is fuelled by material flowing from the cool 'outer' accretion disk. The spin axis of the accretion disk appears to determine the direction of the galaxy's radio jets.