optim_doc: wsolve

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### 2.14 Function wsolve, another linear solver

#### Helptext:

    [x,s] = wsolve(A,y,dy)

    Solve a potentially over-determined system with uncertainty in
    the values.

        A x = y +/- dy

    Use QR decomposition for increased accuracy.  Estimate the
    uncertainty for the solution from the scatter in the data.

    The returned structure s contains

       normr = sqrt( A x - y ), weighted by dy
       R such that R'R = A'A
       df = n-p, n = rows of A, p = columns of A

    See polyconf for details on how to use s to compute dy.
    The covariance matrix is inv(R'*R).  If you know that the
    parameters are independent, then uncertainty is given by
    the diagonal of the covariance matrix, or

       dx = sqrt(N*sumsq(inv(s.R'))')

    where N = normr^2/df, or N = 1 if df = 0.

    Example 1: weighted system

       A=[1,2,3;2,1,3;1,1,1]; xin=[1;2;3];
       dy=[0.2;0.01;0.1]; y=A*xin+randn(size(dy)).*dy;
       [x,s] = wsolve(A,y,dy);
       dx = sqrt(sumsq(inv(s.R'))');
       res = [xin, x, dx]

    Example 2: weighted overdetermined system  y = x1 + 2*x2 + 3*x3 + e

       A = fullfact([3,3,3]); xin=[1;2;3];
       y = A*xin; dy = rand(size(y))/50; y+=dy.*randn(size(y));
       [x,s] = wsolve(A,y,dy);
       dx = s.normr*sqrt(sumsq(inv(s.R'))'/s.df);
       res = [xin, x, dx]

    Note there is a counter-intuitive result that scaling the
    uncertainty in the data does not affect the uncertainty in
    the fit.  Indeed, if you perform a monte carlo simulation
    with x,y datasets selected from a normal distribution centered
    on y with width 10*dy instead of dy you will see that the
    variance in the parameters indeed increases by a factor of 100.
    However, if the error bars really do increase by a factor of 10
    you should expect a corresponding increase in the scatter of
    the data, which will increase the variance computed by the fit.