Example: The set of even nonnegative integers ={, and x=2y, where yeZ} (1)Basic step:+. (2)Recursive step: If neE*, then n+. (3)Closed step: There are no other elements in the set E'except those constructed using steps(1) and (2). Example: (1)Basic step:3∈s. (2)Recursive step: If x and yeS, then x+yeS. (3)Closed step: There are no other elements in the set Sexcept those constructed using steps (1) and (2). S=? S={yly=3x,XEZ*}Example: The set of even nonnegative integers E’={x|x≧0,and x=2y,where yZ} (1)Basic step:0E+ 。 (2)Recursive step: If nE+ ,then n+2E+ . (3)Closed step:There are no other elements in the set E’except those constructed using steps (1) and (2). Example： (1)Basic step:3S。 (2)Recursive step: If x and yS, then x+yS。 (3)Closed step: There are no other elements in the set S except those constructed using steps (1) and (2). S=? S={y|y=3x,xZ+ }