Can someone please explain why they refer to the solution as closed-form? From my understanding, the term closed form would imply that the command constant 'c' can only take values from a finite set? Why so? And how does it defer from Pure proportional navigation, pursuit navigation and LOS navigation in terms of how the solution is closed from or not?
In mathematical terms, an equation is of "closed form" if it can be solved exactly without simplifying assumptions, successive approximations or by for example discarding higher-order terms in a polynomial expansion of one of the state functions. Hence, an equation which admits a closed-form solution can be solved by an explicit algorithm containing a finite number of steps that could be programmed into the feedback block of an automatic control system, for example.