f(7-x)= f(7+x)

F(4-x)= f( 14-x)= f(x)

That is f(x)=f(x+ 10), T= 10,

The closed interval and the [0, 10] function each have two solutions.

Then equation f(x)=0 has 403 roots in the closed interval [0,2011], and equation f(x)=0 has 402 roots in the closed interval [-201,0].

The equation f(x)=0 has 805 roots in the closed interval [-20 1 1, 20 1 1].