I try to do this plot, but when i try do sum, get error. Any suggestion how ti deal with this?

>> suma=sum(-1).^((i-1)/2).*[1-(cosh(i*pi*z)./(2*a))./(cosh((i*pi*b)./(2*a)))].*(cosh((i*pi*y)./(2*a)/i^3))
Error using *
Inner matrix dimensions must agree.
