This paper examines integrated effects of firm heterogeneity and communication network services on international trade. Patterns and effects of trade are analyzed in a general equilibrium model where firms with different productivity levels share among them the cost of network services and compete in a monopolistically competitive market for a differentiated good. The paper reveals that the more efficient country in the production of the differentiated good is not always the net exporter of the good. The less efficient country also has the chance to expand the industry and then to become the net exporter in this intra-industry trade due to the combination of the efficiency effect induced by firm heterogeneity and the cost-sharing effect by the existence of the network service industry. Copyright � 2007 The Authors; Journal compilation � 2007 Blackwell Publishing Ltd.

By calculating the trade indexes (TII, RCA and IIT), mainly in the period 2001-2017, the article analyzes current situation of the dependent relationship and ability to complement each other on trade between Vietnam and China. The results show that Vietnam and China are increasingly becoming important trade partners of each other; Vietnam and China have a particular advantage in the export structure of its exports, showing the trade relations between the two countries are complementary to each other; and the level of intra-industry trade between Vietnam and China is quite high. The US - China trade war broke out will cause the  reversal of the global trade environment. However, in short term, the trade context of the Vietnam - China less affected by this tension. But in the long term, the sanctions are expanded to different sectors will can have unpredictable effects.

Helmet display (HMD) is the core hardware of virtual display technology. It is a small display device mounted on the helmet to produce visual images for the user. The MHD system mainly consists of an image source, optical, and support system. Still, the traditional optical subsystem cannot achieve the functions of the ideal optical system, such as being light-weight, small in size, and having an extensive field angle. This paper established the proper initial model, and the optimal control conditions and surface upgrade strategy were determined. Herein, the design scheme of the free-form prism with good performance was obtained. We solve the feature of large volume and complicated structure of optical components of helmet display and describe the design idea of free-form prism in detail. In this paper, the system resolution we designed is 800 × 600, the Modulation transfer function (MTF) value of the prism is more significant than 0.1 at the spatial resolution of 30lp/mm, and the distortion is less than 5%. The exit pupil diameter of the system is 8mm, and the field angle is 26°×20°.