The document summarizes key economic concepts related to elasticity of demand, international trade, costs of production, and national income. It provides definitions and explanations of price elasticity of demand versus cross elasticity of demand. It also discusses the Heckscher-Ohlin model of international trade and its assumptions. Production costs such as average costs, marginal costs, and cost functions are explained. Finally, the relationship between production, income, and expenditure flows in an economy is outlined in the context of defining national income.