Expressions such as $\cos^2 x$, $\sin^2 x$ are shorthand mathematical notation for $(\cos(x))^2$, $(\sin(x))$2$. In Sage, you have to input them as cos(x)^2, sin(x)^2, as Sage does not parse the above shorthand notation. The error message you are getting is because Sage thinks you want sech to the power 2(k / d ...) and it is telling you that the integer 2 is not "callable", as a function would be (eg if f is a function, you can call f(k / d ...)). Expressions such as$\cos^2 x$,$\sin^2 x$are shorthand mathematical notation for$(\cos(x))^2$,$(\sin(x))$2$.$(\sin(x))^2$.
In Sage, you have to input them as cos(x)^2, sin(x)^2, as Sage does not parse the above shorthand notation.
The error message you are getting is because Sage thinks you want sech to the power 2(k / d ...) and it is telling you that the integer 2 is not "callable", as a function would be (eg if f is a function, you can call f(k / d ...)).