On the geometry of the smallest circle enclosing a finite set of points

A number of numerical codes have been written for the problem of finding the circle of smallest radius in the Euclidean plane that encloses a finite set P of points, but these do not give much insight into the geometry of this circle. We investigate geometric properties of the minimal circle that may be useful in the theoretical analysis of applications. We show that a circle C enclosing P is minimal if and only if it is rigid in the sense that it cannot be translated while still enclosing P. We show that the center of the minimal circle is in the convex hull of P. We use this rigidity result and an analysis of the case of three points to find sharp estimates on the diameter of the minimal circle in terms of the diameter of P.     

黄土丘陵沟壑区沟谷侵蚀演化的坡向差异——以晋西王家沟小流域为例

黄土高原是世界上土壤侵蚀严重的地区之一，年土壤流失量约为22×10⁴t，而坡沟地区的侵蚀，尤其是水沙下坡时坡度对侵蚀尤为严重，坡沟侵蚀中坡度对侵蚀的作用成为现在坡沟侵蚀研究的重要内容，野外考察和前人研究发现，坡向对坡沟侵蚀也起着重要的不可忽视的控制作用。 因此，开展不同坡向土壤侵蚀的研究，对于探讨小流域不同坡向坡沟地貌侵蚀演化特征的定量研究，揭示土壤侵蚀的坡向差异和防治具有重要意义。本文通过对黄土丘陵沟壑区典型小流域王家沟大比例尺航片的解译，在GIS技术支持下，利用沟谷发育度的概念，对研究区内不同坡向的侵蚀差异进行分析研究。研究表明在黄土丘陵沟壑区坡向地貌侵蚀演化差异明显，总体上阳坡的侵蚀强度大于阴坡。各坡向侵蚀强度依次是：南偏西（SW）＞东偏南（ES）＞南向坡（S）＞东向坡（E）＞北偏东（NE）＞西坡（W）。     

对我国生态安全的若干科学思考

本文从分析生态安全的共轭内涵、系统框架、动力学机制和控制论方法入手,探讨区域、城乡和人口生态安全的战略管理和建设方法。认为生态安全的内涵不只是生存稳定性还有发展的支撑能力,不只是环境结构的安全还有生态关系的健全,生态安全不能只用自然生态风险和人类生态胁迫的负面威胁来测度,还要用自然生态服务的正面调节来测度,生态安全不仅可以通过防护修复来保障,还可以通过人工建设来强化。     

关系专用性投资的机会主义效应与治理机制研究——一个独立和互补的调节模型

从强形式机会主义和弱形式机会主义的双重视角,剖析合作创新中关系专用性投资的机会主义效应,探究明确型合同和关系型规范对机会主义效应的独立和互补治理作用。研究结果表明,在合作创新活动中,关系专用性投资不仅会引发强形式机会主义,也会引发弱形式机会主义;明确型合同对强形式机会主义的治理作用更显著,关系型规范对弱形式机会主义的治理作用更显著;明确型合同与关系型规范具有互补性,与单独使用一种治理机制相比,二者同时采用对机会主义效应的治理作用更显著。研究结论从深层次上揭示了合作创新中关系专用性投资的机会主义效应,以及明确型合同和关系型规范2种治理机制的独立和互补作用。     

Beyond Access: Psychosocial Barriers to Computer Literacy

Research consistently indicates that Latinos and African-Americans are less likely to own and use computers than either Caucasians or Asians. While conventional wisdom holds that high computer costs and lack of access are the primary reasons for this discrepancy, my ethnographic research with 100 low-income adults reveals three non-cost-related psychosocial obstacles that significantly undermine motivation for acquiring computer skills: “relevance,” “fear,” and “self-concept.” My research thus suggests that a more complex relationship exists between ethnicity, identity, and attitudes associated with computers than previously examined; this relationship may better explain why some people are choosing not to use computers at this time. My research also indicates that while community technology centers (CTCs) play an important role in helping adults overcome their resistances and achieve computer literacy, they are presently an underutilized resource—less than 5% of San Diegans use them to meet their computing needs. Hence, my research also questions why more residents do not avail themselves of these community resources. I conclude that efforts to increase computer literacy in underserved communities must go beyond physical access and connectivity and consider the role of cultural factors.     

Acrosin activity in spermatozoa of infertile Nigerian males

The objective of this study was to evaluate acrosin activity in spermatozoa of infertile Nigerian men and to compare with those of the fertile men. The acrosin activity was evaluated using the Acroscreen reagent kit. The acrosin activity of the fertile men was 40.7±5.2 mIU/10⁶ sperm (range 30.1–51.3) and those of the infertile men was 22.4±8.33 mIU/10⁶ sperm (range 5.7–39.1). The difference in the mean was statistically significant (P<0.001). There was a subpopulation of the infertile men who had their acrosin activity within normal range of the fertile men, 32.9±1.57 mIU/10⁶ sperm. But the difference in the mean was statistically significant (P<0.001). Acrosin activity decreased with increased morphological changes in the spermatozoa. It is concluded that acrosin activity in the infertile Nigerian men is significantly lower than that in the fertile men. Acrosin activity may also be affected by morphological changes in the spermatozoa.     

Evaluation of ovulisa: An indigenous ELISA kit for detection of ovulation

Aprototype kit for the detection of occurrence of ovulation based on urinary pregnanediol glucuronide estimations by ELISA was developed in our laboratory. This kit was sent for evaluation at 4 centres in Bombay and Pune, India. The diagnosis of ovulation/anovulation based on the results of the kit is comparable with that based on reference parameters commonly used (ultrasonography of ovarian follicles and serum progesterone estimations). The accuracy of the test was 93.5%, specificity was 87.5% and sensitivity was 95.7%. This rapid indigenous test can be very useful in assessing ovulatory status of the woman by the clinicians in their offices.     

黄河三角洲及渤海莱州湾滨海区土壤资源及其合理开发利用

黄河三角洲及渤海莱州湾滨海区土壤含盐量高和肥力低为其突出特点。干旱的气候、黄河尾闾河道的摆动、咸水的侵染、风暴潮的侵袭等自然条件和人类活动决定着土壤发生与发展的总趋势。生物与工种措施相结合综合治理旱、涝、盐、咸、瘦、农、林、牧、渔、盐综合发展，是合理开发4黄河三角洲、土壤资源、土壤盐