设代数系统V1=<A, .>、V2=<A, .>、 V3=<A, ⊙>、 V4=<A, △>，A={ x|x∈Z,1≤x≤20}，

a.b= 2a+b，a.b=max(a, b)，a⊙b=(a+b) mod8 + 2，a△b=min(a,b)，试设计算法并编程实验，

要求：

（1）试判断V1、V2、V3、V4 是否为代数系统。

（2）试判断这些代数系统是否有左单位元与右单位元、零元，如存在，请求出。

-Let Algebra System V1 = <A, .> , V2 = <A, .> , V3 = <A, ⊙> , V4 = <A, △> , A = {x | x ∈ Z, 1 ≤ x ≤ 20}, ab = 2a+b, ab = max (a, b), a ⊙ b = (a+b) mod8+ 2, a △ b = min (a, b ), try to design algorithms and programming experiment requirements: (1) trial judge V1, V2, V3, V4 is an algebraic system. (2) whether the trial judge these algebraic systems have left and right units per unit, nil, if present, the request out.