# Define/store individual elements of Table output for back-substitution

At its core, mine is a very simple communication error between me and Mathematica that I haven't been able to crack, so I do apologise if it seems embarassingly inane.

Please consider the following simplified set-up. I have x[-1] and I seek an expression for x[5] using Table:

root = Table[Solve[2x[b]+x[b-1]+b == 0, x[b]] /.x[-1]->1, {b,0,5}]

The output:

{{{x[0]->-(1/2)}}, {{x[1] -> 1/2 (-1 - x[0])}}, {{x[2] -> 1/2 (-2 - x[1])}},

{{x[3] -> 1/2 (-3 - x[2])}}, {{x[4] -> 1/2 (-4 - x[3])}},

{{x[5] -> 1/2 (-5 - x[4])}}}.

(I don't have the right lingo for this, but) the output elements don't engage like numbers. If I were to try something like root[[1]]+1,

I would get

{{1 + (x[0] -> -(1/2))}}

(and Rules->Equal just changes that -> to =; the behaviour remains the same).

Is it possible to define these x[b] such that back-substitution happens while Table is at work? If not, can you please advise on how to define individual output elements (without directly assigning values from the output to each x[b])?

As an aside, is it better Mathematica practice to abandon this whole Table business and use a loop instead?

--PhysicsHobbit

This seems to be a task for RecurrenceTable:

ClearAll[x, b]
RecurrenceTable[{2 x[b] + x[b - 1] + b == 0, x[-1] == 1}, x, {b, 0, 5}]

{-(1/2), -(1/4), -(7/8), -(17/16), -(47/32), -(113/64)}


Alternatively, you can use RSolve:

ClearAll[func]
func = x /. RSolve[{2 x[b] + x[b - 1] + b == 0, x[-1] == 1}, x, b][[1]];

func /@ Range[0, 5]

 {-(1/2), -(1/4), -(7/8), -(17/16), -(47/32), -(113/64)}


If you have to use Table + Solve you can do

roots = Flatten @ Table[Solve[2 x[b] + x[b - 1] + b == 0, x[b]] /. x[-1] -> 1,
{b, 0,  5}];
Fold[# /. #2 &, roots[[All, 2]], Reverse[roots]]

 {-(1/2), -(1/4), -(7/8), -(17/16), -(47/32), -(113/64)}