So a couple of functions are defined:

f(x,n,a_0,a,b)=a_0 + sum_k=1^n a(k)cos(kx)+b(k)sin(kx)

a_1(k)= **some function of k**

b_1(k)= **some function of k**

The plot should be called as:

f(pi(2x+1),10,1/3,a_1,b_1)

But this yields an error when attempting to plot:

“a_1 is a function. Try using parentheses”