Looping a function that requires previous calculations in order to continue

Question

I am trying to solve a system of overdetermined linear equations for 2 unknowns with 3 variables.

For a given equation of a line, we can write it as: ax + by = c,

which expressed in vector form is Transpose(a)x=c, where a = (a b) and x = (x y).

So far, I've managed to get my code to a point where I can calculate what x1, x2, and x3 are, but I don't know how to loop it in a way that Mathematica will calculate it until the 3 variables converge.

This is what I have done so far: For a list of 3 equations and 2 unknowns:

Subscript[L, 1]: 4x+y=6, Subscript[L, 2]: 5x-y=1, Subscript[L, 3 ]: 2x-3y=4

x0 = {3, 1};
a1 = {4, 1};
x1 = x0 + ((6 - a1.x0)/a1.a1)*a1
a2 = {5, -1};
x2 = x1 + ((1 - a2.x1)/a2.a2)*a2
a3 = {2, -3};
x3 = x2 + ((4 - a3.x2)/a3.a3)*a3

Doing this gives you the points. And to loop it, I thought of defining the equations and then giving it initial parameters to calculate it, but I don't think it works.

eqns = {x1[x0] == x0 + ((6 - a1.x0)/a1.a1)*a1, 
  x2[x1] == x1 + ((1 - a2.x1)/a2.a2)*a2, 
  x3[x2] == x2 + ((4 - a3.x2)/a3.a3)*a3}; inits = {??}

Does anyone have advice? Or whether this kind of question has been asked before?

Answer 1

For one iteration I'd use FoldList:

iter[x_, {a_, c_}] = x + (c - a.x)/a.a a;
A = {{{4, 1}, 6}, {{5, -1}, 1}, {{2, -3}, 4}};
FoldList[iter, {3, 1}, A]

{{3, 1}, {23/17, 10/17}, {79/221, 174/221}, {271/221, -114/221}}

So you can define the process of going from $x_0$ to $x_3$ with

oneloop[x_] := Fold[iter, x, A]

and then nest this oneloop until convergence:

x0inf = FixedPoint[oneloop, {3, 1} // N]

{0.521614, -0.985591}

From this, recover the three points $\{x_1,x_2,x_3\}$ with

Rest@FoldList[iter, x0inf, A]

{{1.67435, -0.697406}, {0.122589, -0.387054}, {0.521614, -0.985591}}

Alternatively, look for an analytic solution for x0inf with

x0inf = {x,y} /. First[Solve[Thread[oneloop[{x,y}]=={x,y}], {x,y}]]

{181/347, -342/347}

from which you get the three points as before,

Rest@FoldList[iter, x0inf, A]

{{581/347, -242/347}, {553/4511, -1746/4511}, {181/347, -342/347}}