# Solving recurrence relationship using iteration

I am trying to create a Mathematica function that basically splits up a continuous function (in this case a physical potential) into steps (strips as it were) and returns the value of the function at that step (i.e., the potential at respective strip boundary). This is used to calculate a transmission coefficient (it's just some function) at every point in that potential.

I have tried to use the in-built function RecurrenceTable, but did not work out (i.e. I get a relation but it's wrong and gives me errors.). I have not fully quit on it, but it would take me too long to explain the inner workings.

Either way, my recurrence relation for some Z is:

Zinput[i] ==  Z0[i]*((Zinput[i + 1] Cosh[k[i] l]) - (Z0[i] Sinh[k[i] l])) /
((Z0[i] Cosh[k[i] l]) - (Zinput[i + 1] Sinh[k[i] l]))


I want to plug this into a Do or For loop so that each value depends on the previous one. This should be easy to achieve, but I have no idea how to set the function to do this.

Just for clarification, the dependence on i is defined as follows:

k[i_] := (Sqrt[2. m (V[i] - En)]/hbar);
Z0[i_] := (- I h  k[i])/m
V[i_] := potential[( i/100.)];
potential[x_] :=  Piecewise[{{((V0/2. )*(Cos[(2. Pi / λ) (x - λ/2)] + 1)),
0 <= x <= M λ}}];


Where h, V0, λ, and M are constants defined by me (basically they scale the potential).

I have attempted to use the nest list function

b[c_] := NestList[Z0[i]*((Zinput[i + 1]Cosh[k[i]l])-(Z0[i]Sinh[k[i]l]))/
((Z0[i] Cosh[k[i] l]) - (Zinput[i + 1] Sinh[k[i] l])), Zinput[i], c]


However, this does not give me something I can plot. I am trying to apply the formula for Zinput (the big long $\sinh$ and $\cosh$ thing) to be the next input while Z0 is the initial value to start with. I also should've mentioned the splitting of the potential is done from right to left (and this is something I cannot change).

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I wrote an answer but then it occurred to me that I was specifying the values in terms of i, while my idea of answer would have worked without i at all. Please clarify why Z0 is a function of i and how it should be updated in i+1, for example. – Peltio Nov 14 '13 at 17:18
Are you saying that input at time i depends on input at time i+1 ?? Normal recurrence is where future values depends on past values, this way the system can be initialized correctly. – Nasser Nov 14 '13 at 17:20
@Nasser, by solving the given eq wrt Zinput[i+1] you get a function of values at index i only. Problem is: how Zo and k vary with i is left unspecified. If explicit forms for such relationships are given, a NestList will help unravel the "i-me" evolution of Zinput. – Peltio Nov 14 '13 at 17:22
Your definition appears to have neither a initial condition (z0 is not defined) nor a possibility for terminating. You define zInput[i] in terms of zInput[i+1], the latter which is not known (nor can it be computed). – DavidC Nov 14 '13 at 18:02
You may wish to examine the way factorial can be defined recursively, and work from there. reference.wolfram.com/mathematica/tutorial/… – DavidC Nov 14 '13 at 18:07

I am not sure to have understood the problem but, if the index i represents the step, you could solve your equation for Zinput[i+1] (which I will call Zin for brevity), getting

Zin[i+1] == Z0[i] * (Z0[i] Sinh[l k[i]] + Zin[i] Cosh[l k[i]]) /
(Z0[i] Cosh[l k[i]] + Zin[i] Sinh[l k[i]])


Then, after defining the value at i=0 or i=1 to stop the recursion, you could compute the next step for Zin with

Zin[i_]:= Z0[i-1] * (Z0[i-1] Sinh[l k[i-1]] + Zin[i-1] Cosh[l k[i-1]]) /
(Z0[i-1] Cosh[l k[i-1]] + Zin[i-1] Sinh[l k[i-1]])


Problem is you did not define how Z0 and k depends on i. If you can give a functional dependance of such variables from i, you could compute all three i-dependent values with a function nextStep[{Zin,Zo,k}] that, given the values at i will give you back the values at i+1. Such function could then be used inside a NestList, along with the respective initial values at i=0 or 1, whatever, and the number of steps (or strips) to take.

Somehow I have this feeling that you might be better off with a NestList.

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