function gamma = hill(x,kvec,sign)
% function: a = hill(x,k,sign)
%   x = series
%   k = vector of tail points to estimate hills at
%   sign = +1 right, -1, left
%   returns vector gamma(j) = shape parameter (gamma) at kvec(j)
%    
if (sign == 0)
    xs = sort(abs(x));
else
    xs = sort(sign*x);
end
n = length(xs);
% xs = abs(xs);

% y = (1/k)* sum( log(xs(n-k+1:n)));
% a = 1/(y-log(xs(n-k+1)));
j = 1;
for k = kvec
    gamma(j) = (1/k) * sum ( log(xs(n-k+1:n))) - log(xs(n-k));
    j=j+1;
end