Application of Linear Sequences to Cryptography

Cryptography is the study of a centuries–old technique of secretly transferring information between parties. Linear recurrences were the chosen method of encryption and decryption in the thesis. The Fibonacci sequence, with its Zeckendorf representation, allows for the flexibility of encoding any number desired based on a particular encoding technique used in the film Sherlock Holmes: A Game of Shadows. The main goal is to find other linear recurrences that possess characteristics similar to the Fibonacci sequence to use as suitable substitutes for encoding. Different sequences were analyzed based on a number of criteria. In order for a sequence to be a candidate, it had to be first deemed a possible sequence based on the nature of the roots of its characteristic equation. Once it passed this test, a particular method was developed for showing that a sequence could be used to encode a set of numbers. This method was applied to various sequences, showing which sequences satisfy the desired encryption method

Twisted versus braided magnetic flux ropes in coronal geometry. I. Construction and relaxation.

We introduce a technique for generating tubular magnetic fields with arbitrary axial geometry and internal topology. As an initial application, this technique is used to construct two magnetic flux ropes that have the same sigmoidal tubular shape, but have different internal structures. One is twisted, the other has a more complex braided magnetic field. The flux ropes are embedded above the photospheric neutral line in a quadrupolar linear force-free background. Using resistive-magnetohydrodynamic simulations, we show that both fields can relax to stable force-free equilibria whilst maintaining their tubular structure. Both end states are nonlinear force-free; the twisted field contains a single sign of alpha (the force-free parameter), indicating a twisted flux rope of a single dominant chirality, the braided field contains both signs of alpha, indicating a flux rope whose internal twisting has both positive and negative chirality. The electric current structures in these final states differ significantly between the braided field, which has a diffuse structure, and the twisted field, which displays a clear sigmoid. This difference might be observable

Substorms on Mercury?

Qualitative similarities between some of the variations in the Mercury encounter data and variations in the corresponding regions of the earth's magnetosphere during substorms are pointed out. The Mariner 10 data on Mercury show a strong interaction between the solar wind and the plant similar to a scaled down version of that for the earth's magnetosphere. Some of the features observed in the night side Mercury magnetosphere suggest time dependent processes occurring there

The Bacterial Photosynthetic Reaction Center as a Model for Membrane Proteins

Membrane proteins participate in many fundamental cellular processes. Until recently, an understanding of the function and properties of membrane proteins was hampered by an absence of structural information at the atomic level. A landmark achievement toward understanding the structure of membrane proteins was the crystallization (1) and structure determination (2-5) the photosynthetic reaction center (RC) from the purple bacteria Rhodopseudomonas viridis, followed by that of the RC from Rhodobacter sphaeroides (6-17). The RC is an integral membrane protein-pigment complex, which carries out the initial steps of photosynthesis (reviewed in 18). RCs from the purple bacteria Rps. viridis and Rb. sphaeroides are composed of three membrane-associated protein subunits (designated L, M, and H), and the following cofactors: four bacteriochlorophylls (Bchl or B), two bacteriopheophytins (Bphe or [phi]), two quinones, and a nonheme iron. The cofactors are organized into two symmetrical branches that are approximately related by a twofold rotation axis (2, 8). A central feature of the structural organization of the RC is the presence of 11 hydrophobic [alpha]-helixes, approximately 20-30 residues long, which are believed to represent the membrane-spanning portion of the RC (3, 9). Five membrane-spanning helixes are present in both the L and M subunits, while a single helix is in the H subunit. The folding of the L and M subunits is similar, consistent with significant sequence similarity between the two chains (19-25). The L and M subunits are approximately related by the same twofold rotation axis that relates the two cofactor branches. RCs are the first membrane proteins to be described at atomic resolution; consequently they provide an important model for discussing the folding of membrane proteins. The structure demonstrates that [alpha]-helical structures may be adopted by integral membrane proteins, and provides confirmation of the utility of hydropathy plots in identifying nonpolar membrane-spanning regions from sequence data. An important distinction between the folding environments of water-soluble proteins and membrane proteins is the large difference in water concentration surrounding the proteins. As a result, hydrophobic interactions (26) play very different roles in stabilizing the tertiary structures of these two classes of proteins; this has important structural consequences. There is a striking difference in surface polarity of membrane and water-soluble proteins. However, the characteristic atomic packing and surface area appear quite similar. A computational method is described for defining the position of the RC in the membrane (10). After localization of the RC structure in the membrane, surface residues in contact with the lipid bilayer were identified. As has been found for soluble globular proteins, surface residues are less well conserved in homologous membrane proteins than the buried, interior residues. Methods based on the variability of residues between homologous proteins are described (13); they are useful (a) in defining surface helical regions of membrane and water-soluble proteins and (b) in assigning the side of these helixes that are exposed to the solvent. A unifying view of protein structure suggests that water-soluble proteins may be considered as modified membrane proteins with covalently attached polar groups that solubilize the proteins in aqueous solution

An Isomorphous Replacement Method for Phasing Twinned Structures

A linear least-squares formulation of the method of isomorphous replacement is presented. With data from untwinned crystals, this approach is shown to be equivalent to the phasing representation developed by Hendrickson & Lattman [Acta Cryst. (1970). B26, 136-143]. A general method for calculating the most probable phase is described and applied to the higher- dimensional problem of phase determination for twinned structures. A method for calculating the best phase with intensity data from twinned crystals is also presented. The dependences of these phasing procedures on the number of derivatives and accuracy of the data sets are evaluated in test calculations

Comparing the influence of ‘describing findings to the examiner’ or ‘examining as in usual practice’ on the students’ performance and assessors’ judgements during physical examination skills assessment

Background: Within assessment of physical examination skills, two approaches are common: “Describing Findings” (students comment throughout); and examining as “Usual Practice” (students only report findings at the end). Despite numerous potential influences on both students’ performances and assessors’ judgements, no prior studies have investigated the influence of either approach on assessments. Methods: Two group, randomised, crossover design. Within a 2-station simulated physical examination OSCE, we manipulated whether students “described findings” or examined as “usual practice”, collecting 1/. performance scores; 2/. Students’/examiners’ cognitive load ratings; ratings of the 3/. fluency and 4/. completeness of students’ presentations and 5/. Students’ task-finishing, comparing all 5 end-points across conditions. Results: Neither students’ performance scores nor examiners’ cognitive load were influenced by experimental condition. Students reported higher cognitive load (7/9) when “describing findings” than “usual practice” (6/9, p=0.002), and were less likely to finish (4 vs 12, p=0.007). Presentation completeness was higher for “describing findings” (mean=2.40, (95CIs=2.05-2.74)) than “usual practice” (mean=1.92 (1.65-2.18),p=0.016), whilst fluency ratings showed a similar trend. Conclusions: The decision to “Describe Findings” or examine as “Usual Practice” does not appear neutral, potentially influencing students’ efficiency, recall and (by inference) learning. Institutions should explicitly select one option based on assessment goals

Cryptic choice of conspecific sperm controlled by the impact of ovarian fluid on sperm swimming behavior

Despite evidence that variation in male–female reproductive compatibility exists in many fertilization systems, identifying mechanisms of cryptic female choice at the gamete level has been a challenge. Here, under risks of genetic incompatibility through hybridization, we show how salmon and trout eggs promote fertilization by conspecific sperm. Using in vitro fertilization experiments that replicate the gametic microenvironment, we find complete interfertility between both species. However, if either species’ ova were presented with equivalent numbers of both sperm types, conspecific sperm gained fertilization precedence. Surprisingly, the species’ identity of the eggs did not explain this cryptic female choice, which instead was primarily controlled by conspecific ovarian fluid, a semiviscous, protein-rich solution that bathes the eggs and is released at spawning. Video analyses revealed that ovarian fluid doubled sperm motile life span and straightened swimming trajectory, behaviors allowing chemoattraction up a concentration gradient. To confirm chemoattraction, cell migration tests through membranes containing pores that approximated to the egg micropyle showed that conspecific ovarian fluid attracted many more spermatozoa through the membrane, compared with heterospecific fluid or water. These combined findings together identify how cryptic female choice can evolve at the gamete level and promote reproductive isolation, mediated by a specific chemoattractive influence of ovarian fluid on sperm swimming behavior

Post-copulatory opportunities for sperm competition and cryptic female choice provide no offspring fitness benefits in externally fertilizing salmon

There is increasing evidence that females can somehow improve their offspring fitness by mating with multiple males, but we understand little about the exact stage(s) at which such benefits are gained. Here, we measure whether offspring fitness is influenced by mechanisms operating solely between sperm and egg. Using externally-fertilising and polyandrous Atlantic salmon (Salmo salar), we employed split-clutch and split-ejaculate in vitro fertilisation experiments to generate offspring using designs that either denied or applied opportunities for sperm competition and cryptic female choice. Following fertilisations, we measured 140 days of offspring fitness after hatch, through growth and survival in hatchery and near-natural conditions. Despite an average composite mortality of 61%, offspring fitness at every life stage was near-identical between groups fertilised under the absence versus presence of opportunities for sperm competition and cryptic female choice. Of the 21,551 and 21,771 eggs from 24 females fertilised under monandrous versus polyandrous conditions, 68% versus 67.8% survived to the 100-day juvenile stage; sub-samples showed similar hatching success (73.1% versus 74.3%), had similar survival over 40 days in near-natural streams (57.3% versus 56.2%), and grew at similar rates throughout. We therefore found no evidence that gamete-specific interactions allow offspring fitness benefits when polyandrous fertilisation conditions provide opportunities for sperm competition and cryptic female choice

Inkjet Fabrication of Frame Dipole FSS

Digital fabrication techniques gives the possibility of producing elements with very thin and precise features which could allow the modification of UHF structures to reduce ink usage while still achieving similar performance. This paper investigates the case where dipole elements are modified into Frame Dipoles by removing areas where the surface current tends to be very low

Interaction of the solar wind with Venus

Two topics related to the interaction of the solar wind with Venus are considered. First, a short review of the experimental evidence with particular attention to plasma measurements carried out on Mariner-5 and Mariner-10 is given. Secondly, the results of some recent theoretical work on the interaction of the solar wind with the ionosphere of Venus are summarized