For each k ≥ 2, let ρk ∈ (0, 1) be the largest number such that there exist k-uniform hypergraphs on n vertices with independent neighborhoods and (ρk + o(1))(kn) edges as n → ∞. We prove that ρk = 1 − 2logk/k + Θ(log log k/k) as k → ∞. This disproves a conjecture of Füredi and the last two authors. 相似文献
A simple, sensitive and economically viable spectrophotometric method for the determination of some Rauwolfia alkaloids (ajmaline, ajmalicine, reserpine and yohimbine-HCl) has been developed. The method involves the oxidation of Rauwolfia alkaloids by iron(III) and subsequent complexation of iron(II) with 1,10-phenanthroline, forming a red-colored complex having the maximum absorbance at 510 nm. The method is applied to the determination of reserpine in tablets of pharmaceutical formulations. The common excipients do not interfere with the proposed method. A statistical comparison of these results with those of a reported method shows good agreement and indicates no significant difference in the precision. 相似文献
The complexes (1-R, 2-R′-indenyl)NiPPh3(thienyl) (R′=H, R=Me (1); Et (2); i-Pr (3); CH2Ph (4); R′=Ph, R=Me (5)) have been prepared and characterized by spectroscopic techniques and, in the case of 1, 2 and 5, by X-ray crystallographic studies. When combined with MAO, these compounds catalyze the polymerization of phenylacetylene to cis-transoidal poly(phenylacetylene) with Mw in the range of 5-7.5×104 Da. NMR studies have revealed that MAO methylates these complexes without ionizing the Nithienyl bond; this implies that the polymerization reactions likely follow a non-cationic mechanism similar to that catalyzed by the corresponding NiCCPh complexes studied previously. Complexes 1-5 reacted with CF3SO3H to produce the corresponding NiOSO2CF3 compounds by protonation at the α-C of the thienyl moiety. The compound (1-Bzindenyl)Ni(PPh3)(OSO2CF3) (9) has been isolated and fully characterized. 相似文献
A family F of k-graphs is called non-principal if its Turán density is strictly smaller than that of each individual member. For each k?3 we find two (explicit) k-graphs F and G such that {F,G} is non-principal. Our proofs use stability results for hypergraphs. This completely settles the question posed by Mubayi and Rödl [On the Turán number of triple systems, J. Combin. Theory A, 100 (2002) 135-152].Also, we observe that the demonstrated non-principality phenomenon holds also with respect to the Ramsey-Turán density as well. 相似文献
A new Cu(II)-ion imprinted polymer (IIP) has been synthesized by copolymerizing salicylic acid and formaldehyde as a monomer and crosslinker, respectively in the presence of Cu(II)-4-(2-pyridylazo) resorcinol complex. The imprinted Cu(II) ions were completely removed by leaching the IIP with 0.05 M EDTA. The maximum adsorption capacity for Cu(II) ions was 310 μg g−1 at pH 6. The IIP was repeatedly used in adsorption–desorption experiments for seven times with recoveries ~95%. The relative selectivity factor (αr) values of Cu(II)/Zn(II), Cu(II)/Cd(II), Cu(II)/Ni(II) and Cu(II)/Co(II) are 3.17, 2.90, 2.47 and 3.37, respectively. The detection limit corresponding to three times the standard deviation of the blank was found to be 3.0 μg L−1. The developed IIP has also been tested for preconcentration and recovery of Cu(II) ions from water samples.
f (l,k) be the minimum n with the property that every coloring yields either with , or with all distinct. We prove that if , then as . This supports the conjecture of Lefmann, R?dl, and Thomas that .
Received July 2, 1998 相似文献