sestck(x,start,c,k,tol)

function scale = sestck(x,start,c,k,tol)

% Computes Tukey's biweight objectief function (scale) corresponding 
% with the mahalanobis distances x.  

if start>0 
    s = start;
else y=abs(x);
    [n,p]=size(x);
    if  ((n==1) & (p>1))
        x = x'; [n,p] = size(x);
    end
    x = sort(x);
    if  floor(n/2) == n/2
        s = (x(n/2,:) + x((n+2)/2,:))/2;
    else
        s = x((n+1)/2,:);
    end
    s = s/0.6745;
end
crit = 2*tol;
rhoold = mean(rhobiweight(x/s,c))-k;
while crit >= tol
    delta = rhoold/mean(psibiweight(x/s,c).*(x/(s^2)));
    isqu = 1; okay = 0;
    while ((isqu<10) & (okay~=1))
        rhonew = mean(rhobiweight(x/(s+delta),c))-k;
        if abs(rhonew) < abs(rhoold)
            s = s+delta;
            okay = 1;
        else
            delta = delta/2;
            isqu = isqu+1;
        end     
    end
    if isqu == 10
        crit = 0;
    else
        crit = (abs(rhoold)-abs(rhonew))/(max([abs(rhonew),tol]));
    end
    rhoold = rhonew;
end
scale = abs(s);