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161.
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164.
This paper, of which a preliminary version appeared in ISTCS'92, is concerned with generalized network flow problems. In a generalized network, each edgee = (u, v) has a positive flow multipliera
e
associated with it. The interpretation is that if a flow ofx
e
enters the edge at nodeu, then a flow ofa
e
x
e
exits the edge atv.
The uncapacitated generalized transshipment problem (UGT) is defined on a generalized network where demands and supplies (real numbers) are associated with the vertices and costs (real numbers) are associated with the edges. The goal is to find a flow such that the excess or deficit at each vertex equals the desired value of the supply or demand, and the sum over the edges of the product of the cost and the flow is minimized. Adler and Cosares [Operations Research 39 (1991) 955–960] reduced the restricted uncapacitated generalized transshipment problem, where only demand nodes are present, to a system of linear inequalities with two variables per inequality. The algorithms presented by the authors in [SIAM Journal on Computing, to appear result in a faster algorithm for restricted UGT.Generalized circulation is defined on a generalized network with demands at the nodes and capacity constraints on the edges (i.e., upper bounds on the amount of flow). The goal is to find a flow such that the excesses at the nodes are proportional to the demands and maximized. We present a new algorithm that solves the capacitated generalized flow problem by iteratively solving instances of UGT. The algorithm can be used to find an optimal flow or an approximation thereof. When used to find a constant factor approximation, the algorithm is not only more efficient than previous algorithms but also strongly polynomial. It is believed to be the first strongly polynomial approximation algorithm for generalized circulation. The existence of such an approximation algorithm is interesting since it is not known whether the exact problem has a strongly polynomial algorithm.Corresponding author. Research was done while the first author was attending Stanford University and IBM Almaden Research Center. Research partially supported by ONR-N00014-91-C-0026 and by NSF PYI Grant CCR-8858097, matching funds from AT&T and DEC.Research partially supported by ONR-N00014-91-C-0026. 相似文献
165.
166.
Edith Bertling-Kampf Michael Heimann Rainer Kleibrink Heinrich C. Marsmann 《无机化学与普通化学杂志》1993,619(9):1639-1645
Silicate Anions in Alkali Silicate Melts Melts of alkali silicates with molar ratios of alkali (Li, Na, K, Cs) to silicon between R = 4.0 and ca. 2.0 were prepared, quenched and worked up to the trimethylsilyl silicic acid esters. These were identified by comparison to the GC and silicon-29 NMR data of trimethylsilyl silicic acid esters from other sources. It was found that with cations of Li and Na mostly linear silicates were formed. But with the cations of K and Cs a considerable amount of the cyclic species Si6O96? was present. Branched silicates were of minor importance only. Besides the alkali silicon ratio, the temperature of the melt before quenching influences the composition of the silicate mixture. 相似文献
167.
Structure of the Valenciaxanthins and Valenciachromes Valenciaxanthin, a carotenoid first isolated from Californian Valencia orange juice in 1952/1954 by American scientists, was re-isolated from fresh Spanish ‘Navelinas’ and shown, by spectroscopical and chiroptical examination, to have the unexpected (9Z)-10′-apo-11′,12′-dihydroviolaxantin-10′-ol structure 6 . A further, very minor component represents the (all-E)-structure 7 . Therefore, the Valenciachromes are the furanoid rearrangement products of 6 and 7 and, thus, stereoisomers of the 10′-apo-11′,12′-dihydroauroxanthiiv 10′-ols 8 . Valenciaxanthin represents a modification of the common cleavage of carotenoids in higher plants according to the scheme C40 → C27 + C13, insofar as the reduction step not only includes the aldehyde function but also the subsequent conjugated double bond. 相似文献
168.
Frank R. Prince Edith A. Turi Eli M. Pearce 《Journal of polymer science. Part A, Polymer chemistry》1972,10(2):465-470
Copolyamides prepared from cis- and trans-1,4-bisaminomethylcyclohexane with adipic, sebacic, and dodecanoic acids showed a decrease in melting point with increasing cis diamine content and increasing chain length of the diacid. Most unusual was the fact that the glass transition temperature of these copolymers were essentially independent of diamine isomer content. The existence of an intermediate conformation for some of the cyclohexane rings such as the “twistmer” is suggested to account for this discrepancy. The Tg, Tc, and Tm all decreased with increasing chain length of the diacid. The thermal stability as exemplified by the polysebacamides, was independent of diamine isomer content. 相似文献
169.
W. Pilz und Edith Stelzl 《Fresenius' Journal of Analytical Chemistry》1968,240(1):23-31
Zusammenfassung Die Test-Pack-Methode zur Bestimmung der Glucose-6-phosphat-Dehydrogenase von Boehringer wurde zu einer exakten analytischen Methode umgearbeitet, die auch höchsten Ansprüchen genügt. Die wesentlichsten Neuerungen sind die Wahl von Hämoglobin als Bezugsgröße, die Vergrößerung der tatsächlich gemessenen Extinktionsdifferenz von 0,004–0,005 auf 0,200–0,500, womit eine wesentlich höhere Sicherheit und Genauigkeit erreicht wird, sowie eine praktikablere Bereitung des Hämolysates, das von Stromata frei ist, wodurch Störungen durch Trübungen während der Reaktion vermieden werden. Die Erythrocyten werden 5mal gewaschen, wodurch das im Serum vorhandene Enzym 6-Phosphogluconsäure-Dehydrogenase, das ebenfalls NADPH liefert, vollkommen ausgewaschen wird und damit nicht mehr stören kann. Der G-6-PDH-Gehalt sehwankt von Mensch zu Mensch sehr stark, bleibt aber bei ein und demselben Menschen über lange Zeit konstant. Die Normalwerte liegen bei 320 und 640 U/mg Hb. Die Fehlergrenze der Methode liegt bei ± 0,5 U/mg Hb.
VI. Mitt.: Pilz, W., u. I. Johann: diese Z. 218, 426 (1966).
Heinrich Hörlein zum Geburtstag gewidmet. 相似文献
Special analytical methods for biochemistry and physiological chemistryPart VII. Analytical determination of glucose-6-phosphate-dehydrogenase (G-6-PDH)
The test-pack method for the determination of glucose-6-phosphate-dehydrogenase according to Boehringer was modified to give an exact analytical method which meets even the highest of requirements. The most important changes are: the choice of haemoglobin as a reference, the increase of actually measured difference in absorbance from 0.004–0.005 to 0.200–0.500 by which a much greater reliability and accuracy is obtained, as well as a more practicable preparation of an haemolysate, free of stomata, whereby interference by turbidities during reaction are avoided. The erythrocytes are washed five times whereby the enzyme 6-phosphogluconic acid-dehydrogenase contained in the serum which forms also NADPH, is washed out completely and consequently cannot more be an interfering factor. The G-6-PDH content varies very much from man to man, however it remains constant over a long period of time in one and the same man. The normal values are 320 and 640 U/mg Hb. The limit of error is ±0.5 U/mg Hb.
VI. Mitt.: Pilz, W., u. I. Johann: diese Z. 218, 426 (1966).
Heinrich Hörlein zum Geburtstag gewidmet. 相似文献
170.
Preferential adsorption of allopurinol (1H-pyrazolo-[3,4-d]-pyrimidin-4-ol) at graphite electrodes from pH 7.0 phosphate buffer solutions of 2,6-diaminopurine and 2,6-diamino-8-purinol allowed in situ electrode modification. Modified electrodes were applied to the simultaneous determination of 2,6-diaminopurine and 2,6-diamino-8-purinol in phosphate buffer at pH 7.0. Differential pulse polarography allowed both compounds to be quantified in the concentration range 1 × 10?6–5 × 10?4 M. The relative standard deviation of the peak current is about 10%. Calibration curve characteristics are C (μM) = 0.0094 ± 0.0002 I (μA) + 0.22 ± 0.07 μA, with r = 0.9985 and C (μM) = 0.0065 ± 0.0001 I (μA) + 0.03 ± 0.04 μA with r = 0.9990 for 2,6-diamino-8-purinol and 2,6-diaminopurine, respectively. 相似文献