function  [r, dr] = correxp(theta, d)%CORREXP  Exponential correlation function%%           n%   r_i = prod exp(-theta_j * |d_ij|)%          j=1%% If length(theta) = 1, then the model is isotropic: % theta_j = theta(1), j=1,...,n%% Call:    r = correxp(theta, d)%          [r, dr] = correxp(theta, d)%% theta :  parameters in the correlation function% d     :  m*n matrix with differences between given data points% r     :  correlation% dr    :  m*n matrix with the Jacobian of r at x. It is%          assumed that x is given implicitly by d(i,:) = x - S(i,:), %          where S(i,:) is the i'th design site. % hbn@imm.dtu.dk  % Last update April 12, 2002[m n] = size(d);  % number of differences and dimension of datalt = length(theta);if  lt == 1,  theta = repmat(theta,1,n);elseif  lt ~= n  error(sprintf('Length of theta must be 1 or %d',n))else  theta = theta(:).';endtd = abs(d) .* repmat(-theta, m, 1);r = exp(sum(td,2));if  nargout > 1  dr = repmat(-theta,m,1) .* sign(d) .* repmat(r,1,n);end