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vacuum
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  vacuum
Given a complete graph with vertex setX and subsetsX1,X2,…,Xn, the problem of finding a subgraphG with minimum number of edges such that for everyi=1,2,…,n, G contains a spanning tree onXi, arises in the design of vacuum systems.
      
The results also show that even under these conditions, neither vacuum states nor concentration states can be formed in finite time.
      
It was found that the optimum conditions of the preparation are hydrothermal crystallization for 8 h at 180°C, followed by vacuum drying at 45°C and calcination at 500°C for 2 h.
      
The 185 nm vacuum UV (VUV) intensity of the MWUVL amounted to 12% of the total UV output, about 1.7 times as high as that of the con ventional low-pressure mercury lamp.
      
Chinese fir plantation sapwood and heartwood boards were treated by three drying methods: radio frequency-vacuum drying (RFVD), conventional kiln drying (KD) and high temperature drying (HTD).
      
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4-Methyl-2,6-dichloropyrimidine was prepared by heating 4-methyl-uracil with phosphorous oxychloride in the presence of phosphorous pentachloride; and it boiled at 97° at 7 mm, at 102° at 10 mm, or at 113° at 13 mm. 4-Methyl-2,6- dichloropyrimidine reacted with sodium methoxide in anhydrous methyl alcohol, forming 4-methyl-2,6- dimethoxy-pyrimidine, which was isolated by ether extraction and pnrified by vacuum distillation. Pure 4-methyl-2,6-dimethoxy-pyrimidine boiled at 85-87° at 7 mm, or at 103° at...

4-Methyl-2,6-dichloropyrimidine was prepared by heating 4-methyl-uracil with phosphorous oxychloride in the presence of phosphorous pentachloride; and it boiled at 97° at 7 mm, at 102° at 10 mm, or at 113° at 13 mm. 4-Methyl-2,6- dichloropyrimidine reacted with sodium methoxide in anhydrous methyl alcohol, forming 4-methyl-2,6- dimethoxy-pyrimidine, which was isolated by ether extraction and pnrified by vacuum distillation. Pure 4-methyl-2,6-dimethoxy-pyrimidine boiled at 85-87° at 7 mm, or at 103° at 13 mm, and melted at 62-65°. It was recrystallized from petroleum ether, m.p. 65-66°. In the above reaction, there was isolated a white solid, suspending in the ethereal solution and being collected separately. This white solid, considered as a by-product, was dissolved in hot water and acidified with acetic acid, whereupon it separated in needles. After recrystallization from water, it melted at 201-202°. It was tentatively assigned to be 4-methyl-2-methoxy- uracil. Further, 2,6-dialkoxy-pyrimidines were prepared similarly as 4-methyl-2,6-dimethoxy- pyrimidine: 4-Methyl-2,6-dichloropyrimidine reacted with sodium ethylate in anhydrous ethyl alcohol, forming 4-methyl-2,6-diethoxy-pyrimidine, which boiled at 110°/11 ram. 4-Methyl- 2,6-dichloropyrimidine was treated with sodium n-propoxide in normal propyl alcohol, forming 4-methyl-2,6-di-n-propoxy-pyrimidine, which boiled at 120°/5 mm. 4-Methyl- 2,6-dichloropyrimidine reacted with sodium iso-propoxide in isopropyl alcohol, forming 4-methyl-2,6-di-isopropoxy-pyrimidine, which boiled at 103°/3 mm. The action of sodium n-butoxide in normal butyl alcohoI upon 4-methyl-2,6-dichloropyrimidnie gave 4-methyl- 2,6-di-n-butoxy-pyrimidine, which boiled at 147-148°/5 mm. The action of sodium isobutoxide in isobutyl alcohol upon 4-methyl-2,6-dichloropyrimidine gave 4-methyl-2,6- isobutoxy-pyrimidine, which boiled at 132-133°/6 mm. The action of sodium isopentoxide in isopentyl alcohol upon 4-methyl-2,6-dichloropyrimidine gave 4 methyl-2,6-di-isopentoxy- pyrimidine, which boiled at 145-146°/3 mm. 4-Methyl-2,6-dichloropyrimidine reacted with sodium benzoxide in benzyl alcohol, giving 4-methyl-2,6-dibenzoxy-pyrimidine, which boiled at 231°/6 mm. 4-Methyl-2,6-dimethoxy-pyrimidine was heated in a sealed tube at 330-350°, giving the completely rearranged isomeric compound, 1,3,4-trimethyl-uracil, which was purified by vacuum sublimation at 130° at 10 mm and then by recrystallization from 95% alcohol. The latter melted at 107-109°. Nevertheless, 4-methyl-2,6-dimethoxy-pyrimidine was dis- solved in methyl iodide, and kept at room temperature in the dark with occasional shaking; whereupon the partially rearranged product, 2-oxy-3,4-dimethyl-6-methoxy-pyrimidine, gradually separated out. After recrystallization from absolutealcohol, it melted at 134-135.5°. Its structure was established as follows: Pure 2-oxy-3,4-dimethyl-6-methoxy-pyrimidine was heated with dilute hydrochloric acid for one hour; whereupon 3,4-dimethyl-uracil, m.p. 220-221°, separated out. This partially rearranged product, 2-oxy-3,4-dimethyl-6-methoxy- pyrimidine was heated at 335-350° and was again transformed into its stable and completely rearranged modification, 1,3,4-trimethyl-uracil, which was purified by vacuum sublimation and then recrystallization from 95% alcohol. The latter melted at 109-110°.

(1)4-甲基-2,6-二氯代嘧啶與鈉醇和醇的溶液作用,可以形成相應的2,6-二烴氧基嘧啶。 (2)4-甲基-2,6-二甲氧基嘧啶加熱至高温度即可轉變成其穩定結構的異構體1,3,4-三甲基-2,6-二羥基嘧啶。另一方面,在碘代甲烷催化劑的影響下,部份轉變成2-氧代-3,4-二甲基-6-甲氧基嘧啶;此化合物加熱卽可發生完全的轉變作用而形成其異構體1,3,4-三甲基-2,6-二羥基嘧啶。

With the rapid progress of the electronic art, there has been a steadily increasing demand for still wider-bandwidth amplifiers. The recent introduction of distributed amplification concept has provided a new technique and powerful means for designing amplifiers with top cutoff frequencies far in excess of those previously obtainable with,ordinary amplifier circuits. A distributed amplifier of the low-pass type can easily be constructed to have a uniform frequency response from audio frequencies to frequencies...

With the rapid progress of the electronic art, there has been a steadily increasing demand for still wider-bandwidth amplifiers. The recent introduction of distributed amplification concept has provided a new technique and powerful means for designing amplifiers with top cutoff frequencies far in excess of those previously obtainable with,ordinary amplifier circuits. A distributed amplifier of the low-pass type can easily be constructed to have a uniform frequency response from audio frequencies to frequencies as high as several hundred me using conventional vacuum tubes. Unlike conventional circuits, distributed amplifiers have an attainable gain-bandwidth product which is not limited by shunt capacitance associated with the vacuum tubes and circuit wiring; the high-frequency limit being determined entirely by high-frequency effects within the tube proper.The purpose of this paper is to describe the basic principle of distributed amplification and to show how such an amplifier employing various types of transmission lines may be designed. Practical methods hi design and design details are given for a three-stage distributed amplifier, using fourteen 6AK5 pentodes with a frequency response of 0.1 mo to 140 mo and a gain of 33±1 db.Both the negative mutual-inductance m-derived and constant K artificial delay lines hare been used. The former offers the advantage of a more linear phase characteristic and a more uniform response both in amplitude and delay time.The experimental results corroborate the predictions based on the first-order theory described in this paper.

分布式放大是最近宽频带放大的最大成就,过去多年来电子学所应用的各种宽频带放大方法,其高频部分因受电子管电容和线路的分布电容所限制,不能获得理想的结果,而利用分布式放大的理论,所制成的宽频带放大器,远较一般普通的宽频带放大器,有更为宽阔的频带;从它的设计和构造上来看,也较负反馈的宽频带放大器为简单。制造一架自数千周至数百兆周的分布式放大器,在技术上并没有很大困难。本文拟对分布式放大的原理作扼要的分析;并提出了采用各类型仿真线所构成的分布式放大器的设计方法,并利用该设计方法,试作了一只三级十四管的分布式放大器,其增益为33±<1分贝,频宽自100千周至140兆周。由实验结果证明,采用m导出式低通滤波器所构成的仿真线的分布式放大器,实较用常K式者,具有更佳的相移特性和频率特性,这与理论上的分析是一致的。

The title compound was synthesized from D-xylose.Methyl α-and β-D-xylofuranoside mixture(Ⅱ),prepared according to Levene et al.was treated with toluene-p-sulphonyl chloride inpyridine first at 0℃ and then at room temperature for four days,followed by usual procedureof isolation and fractional crystallization from alcohol to give 45.1% yield of crystalline methyl2,3,5-tri-O-toluene-p-sulphonyl-β-D-xylofuranoside(Ⅲ),m.p.131—132℃,[α]_D~(24.5)=-31.5°(chloroform,c=1.00,l=2 dm.).In order to obtain(Ⅲ)in crystalline...

The title compound was synthesized from D-xylose.Methyl α-and β-D-xylofuranoside mixture(Ⅱ),prepared according to Levene et al.was treated with toluene-p-sulphonyl chloride inpyridine first at 0℃ and then at room temperature for four days,followed by usual procedureof isolation and fractional crystallization from alcohol to give 45.1% yield of crystalline methyl2,3,5-tri-O-toluene-p-sulphonyl-β-D-xylofuranoside(Ⅲ),m.p.131—132℃,[α]_D~(24.5)=-31.5°(chloroform,c=1.00,l=2 dm.).In order to obtain(Ⅲ)in crystalline state,the methyl D-xylofuranoside mixture should be distilled under a vacuum not below 0.05 mm.Methyl β-D-xylopyranoside(Ⅷ)was similarly converted into methyl 2,3,4-tri-O-toluene-p-sulphonyl-β-D-xylopyranoside(Ⅸ),m.p.138—139℃,[α]_D~24=-37°(chloroform,c=0.50,l=2 dm.)with a yield of 81%.These two tritoluene-p-sulphonyl esters showed a mixed melting pointof 120—123℃.On the other hand,the authentic methyl 2-O-toluene-p-sulphonyl-β-D-xylofura-noside obtained by hydrolysis of the crystalline methyl 2-O-toluene-p-sulphonyl-3,5-O-isopropyli-dene-β-D-xylofuranoside as reported in Part Ⅳ on similar treatment gave rise to a tritoluene-p-sulphonyl ester identical in every respect with(Ⅲ),and so established the anomeric configu-ration of the latter.(Ⅲ)was caused to react with sodium iodide in acetone at 100℃(Oldham-Rutherfordreaction)for eight hours to give 97% yield of methyl 2,3-di-O-toluene-p-sulphonyl-5-deoxy-5-iodo-β-D-xylofuranoside(Ⅳ),m.p.105—106℃,[α]_D~(23.5)=-55°(chloroform,c=1.00,l=2dm.),which was in turn hydrogenated in the presence of Raney nickel to yield the syrupy methyl2,3-di-O-toluene-p-sulphonyl-β-D-xylomethyloside(Ⅴ).Finally,(Ⅴ)was treated with sodiummethoxide in chloroform at 0℃ and methyl 2,3-anhydro-β-D-ribomethyloside was obtained as acolorless,mobile liquid,b.p.46—48℃/1—2 mm,[α]_D~24=-113.5°(chloroform,c=0.52,l=2 dm.).The overall yield from D-xylose was 18%.The structure of the anhydro sugar wasdeduced from existing evidence.

自木糖经过五步反应合成了2,3-内醚-5-去羟-β-D-核糖甲基甙(Ⅵ)。这五步反应是:D-木糖(Ⅰ)→D-木糖甲基呋喃甙(Ⅱ,α-和β-端基差向异构体混合物)→2,3,5-三-O-对甲苯磺酰基-β-D-木糖甲基呋喃甙(Ⅲ)→2,3-二-O-对甲苯磺酰基-5-去羟-5-碘代-β-D-木糖甲基呋喃甙(Ⅳ)→2,3-二-O-对甲苯磺酰基-5-去羟-β-D-木糖甲基甙(Ⅴ)→2,3-内醚-5-去羟-β-D-核糖甲基甙(Ⅵ)。总产率是18%。将已知的结晶的2-O-对甲苯磺酰基-β-D-木糖甲基呋喃甙(Ⅶ)进行对甲苯磺酰化,得到与上述完全相同的Ⅲ,这样就证明了Ⅲ的端基的构型。同时,也从β-D-木糖甲基吡喃甙(Ⅷ)制备了2,3,4-三-O-对甲苯磺酰基-β-D-木糖甲基吡喃甙(Ⅸ)。根据现有的知识肯定了合成的内醚糖的结构。

 
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