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  • Articles  (164)
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  • 1
    ISSN: 1617-4623
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Mutants of Aspergillus nidulans blocked in the main pathway of cysteine synthesis show an elevated level of the enzymes involved in the synthesis of cysteine from homocysteine i.e. cystathionine β-synthase and γ-cystathionase and a depressed level of homocysteine methyltransferase. This results in a considerable change in the sulfur amino acids pool as compared to the wild type. Upon addition of cysteine to the growth medium the first two enzymes are repressed while the level of the third one increases. These data indicate that the two diverging pathways of homocysteine metabolism are anti-coordinately regulated.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Aequationes mathematicae 27 (1984), S. 150-156 
    ISSN: 1420-8903
    Keywords: Primary 10A50, 39A10, 10L20 ; Secondary 39A99
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The local behavior of the iterates of a real polynomial is investigated. The fundamental result may be stated as follows: THEOREM. Let xi, for i=1, 2, ..., n+2, be defined recursively by xi+1=f(xi), where x1 is an arbitrary real number and f is a polynomial of degree n. Let xi+1−xi≧1 for i=1, ..., n + 1. Then for all i, 1 ≦i≦n, and all k, 1≦k≦n+1−i, $$ - \frac{{2^{k - 1} }}{{k!}}〈 f\left[ {x_1 ,... + x_{i + k} } \right]〈 \frac{{x_{i + k + 1} - x_{i + k} + 2^{k - 1} }}{{k!}},$$ where f[xi, ..., xi+k] denotes the Newton difference quotient. As a consequence of this theorem, the authors obtain information on the local behavior of the solutions of certain nonlinear difference equations. There are several cases, of which the following is typical: THEOREM. Let {xi}, i = 1, 2, 3, ..., be the solution of the nonlinear first order difference equation xi+1=f(xi) where x1 is an arbitrarily assigned real number and f is the polynomial $$f(x) = \sum\limits_{j = 0}^n {a_j x^j } ,n \geqq 2$$ . Let δ be positive with δn−1=|2n−1/n!an|. Then, if n is even and an〈0, there do not exist n + 1 consecutive increments Δxi=xi+1−xi in the solution {xi} with Δxi≧δ. The special case in which the iterated polynomial has integer coefficients leads to a “nice” upper bound on a generalization of the van der Waerden numbers. Ap k -sequence of length n is defined to be a strictly increasing sequence of positive integers {x 1, ...,x n } for which there exists a polynomial of degree at mostk with integer coefficients and satisfyingf(x j )=x j+1 forj=1, 2, ...,n−1. Definep k (n) to be the least positive integer such that if {1, 2, ...,p k (n)} is partitioned into two sets, then one of the two sets must contain ap k -sequence of lengthn. THEOREM. pn−2(n)≦(n!)(n−2)!/2.
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  • 3
    ISSN: 1434-6079
    Keywords: 36.40 ; 64.70
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The evolution of thermodynamic, dynamic and quantum size effects in clusters is expected to contribute towards the merging between microscopic and macroscopic points of view in molecular, surface and bulk phenomena.
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  • 4
    ISSN: 1435-5914
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract A family ℬ of sequences has the Ramsey property if for every positive integerk, there exists a least positive integerf ℬ(k) such that for every 2-coloring of {1,2, ...,f ℬ(k)} there is a monochromatick-term member of ℬ. For fixed integersm 〉 1 and 0 ≤q 〈 m, let ℬq(m) be the collection of those increasing sequences of positive integers {x 1,..., xk} such thatx i+1 − xi ≡ q(modm) for 1 ≤i ≤ k − 1. Fort a fixed positive integer, denote byA t the collection of those arithmetic progressions having constant differencet. Landman and Long showed that for allm ≥ 2 and 1 ≤q 〈 m, ℬ q(m) does not have the Ramsey property, while ℬ q(m) ∪A m does. We extend these results to various finite unions of ℬ q(m) 's andA t 's. We show that for allm ≥ 2, $$ \cup $$ q=1 m−1 ℬq(m) does not have the Ramsey property. We give necessary and sufficient conditions for collections of the form ℬ q(m) ∪ ( $$ \cup $$ t ∈ T A t) to have the Ramsey property. We determine when collections of the form ℬa(m1) ∪ ℬb(m2) have the Ramsey property. We extend this to the study of arbitrary finite unions of ℬq(m)'s. In all cases considered for whichℬ has the Ramsey property, upper bounds are given forf ℬ.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Graphs and combinatorics 14 (1998), S. 131-142 
    ISSN: 1435-5914
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract.  A quasi-progression of diameter n is a finite sequence for which there exists a positive integer L such that for . Let be the least positive integer such that every 2-coloring of will contain a monochromatic k-term quasi-progression of diameter n. We give a lower bound for in terms of k and i that holds for all . Upper bounds are obtained for in all cases for which . In particular, we show that . Exact formulae are found for and . We include a table of computer-generated values of , and make several conjectures.
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  • 6
    ISSN: 1436-5065
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geography , Physics
    Notes: Summary Wave-organized convective features in the southwest Indian Ocean are described using Hovmoller composites of satellite imagery, OLR anomalies and ECMWF precipitable water departures during the southern summer. Westward movement of large convective elements is noted in the 10–20°S latitude band in about half of the years between 1970 and 1984. A study of 47 convective systems from satellite imagery establishes the climatological features, including zonal propagation speeds for maritime systems in the range −2 to −4 m s−1, wavelengths of 25–35° longitude (3,000 km), lifespans of 10–20 days and convective areas of 7–10° longitude (800 km). Transient convective waves over the tropical SW Indian Ocean are slower and more diverse than their northern hemisphere counterparts. Interannual tendencies in the frequency and mode are studied. Wet summers over SE Africa correspond with an increased frequency of westward moving convective systems, whereas in dry summers convective systems tend to be quasi-stationary. INSAT data composites provide additional insight into the convective structure and show that tropical waves penetrated into southern Africa in February 1988. A more quantitative assessment of transient convective waves is provided by Hovmoller composites of OLR anomalies and precipitable water departures. Both display westward moving systems in 1976 and 1984 and highlight the wide variety and mixed mode character of convective waves. A case study is analyzed which illustrates the deepening of a moist, unstable layer coincident with the westward passage of a convective wave.
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  • 7
    ISSN: 1436-5065
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geography , Physics
    Notes: Summary Differences in the convective potential of troughs passing over the plateau of southern Africa in the early summer are assessed using operational synoptic weather data and radiosonde time-height sections. Wet and dry trough cases are chosen on the basis of the intensity and distribution of rainfall, sharp thermodynamic changes across the plateau and the passage of a geopotential wave. Composite differences are computed and indicate a high-low geopotential anomaly in the east-west direction, and a threefold increase in precipi-table water from 15 mm in dry cases to 28 mm in wet cases. The 500 hPa structure obtained by differencing wet and dry composites is dominated by low geopotentials and cyclonic vorticity over the plateau near 25°S, 25°E, and high geopotentials and anticyclonic vorticity to the south over the oceans near 40°S, 30°E. The dipole anomaly suggests a diffluent tilted baroclinic wave in the subtropical jet stream in convective cases. A double jet streak structure in the wet events enhances upper divergence which contributes to widespread uplift in the mid-troposphere. A case study comparison highlights the importance of pre-frontal moist influx, the kinematic trigger and thermodynamic instability.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1435-5914
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract The number of partitions of a bi-partite number into at mostj parts is studied. We consider this function,p j (x, y), on the linex+y=2n. Forj≤4, we show that this function is maximized whenx=y. Forj〉4 we provide an explicit formula forn j so that, for alln≥n j ,x=y yields a maximum forp j (x,y).
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Graphs and combinatorics 2 (1986), S. 351-356 
    ISSN: 1435-5914
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract Certain generalizations of arithmetic progressions are used to define numbers analogous to the van der Waerden numbers. Several exact values of the new numbers are given, and upper bounds for these numbers are obtained. In addition, a comparison is made between the number of different arithmetic progressions and the number of different generalized arithmetic progressions.
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  • 10
    ISSN: 1435-5914
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract Numbers similar to those of van der Waerden are studied. We consider increasing sequences of positive integers {x 1,x 2,...,x n } that either form an arithmetic progression or for which there exists a polynomialf with integer coefficients and degree exactlyn − 2, andx j+1 =f(x j ). We denote byq(n, k) the least positive integer such that if {1, 2,...,q(n, k)} is partitioned intok classes, then some class must contain a sequence of the type just described. Upper bounds are obtained forq(n, 3), q(n, 4), q(3, k), andq(4, k). A table of several values is also given.
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