The substitution elasticity tends to 1. According to the query information, the substitution elasticity of CD function is 1, and when CES function tends to CD function, its substitution elasticity is 1. Function, mathematical terminology. Its definition is usually divided into traditional definition and modern definition. The essence of these two functional definitions is the same, but the starting point of narrative concept is different. The traditional definition is from the perspective of movement change, and the modern definition is from the perspective of set and mapping.