*3.2.1.3 Characteristic variable Δ*en*•*Δen�*1*

The product of two adjacent error changes is defined as the characteristic variable that characterizes the extreme state of the error. If *Δen•Δen*�1 < 0, it means that the extreme value appears; if *Δen•Δen*�1 > 0, it means that there is no extreme value. The combination of the characteristic variables *Δen•Δen*�1 and *en•Δen*�1 can determine the change trend of the dynamic process when the error has an extreme value. As shown in **Figure 19**, extreme values appear at points B and C ', But their *en•Δen* values have opposite signs like below:

Point B: *Δen•Δen*�1 < 0, the error of *en•Δen* tends to decrease after point B. Point C<sup>0</sup> : *Δen•Δen*�1 < 0, *en•Δen* > 0, the error gradually increases after point C<sup>0</sup> .
