function [UR,DS]=extract_eigenfunctions(U,V,OBS,DEPTH,l,r,R);
% MATLAB HELP:
%
% function [UR,DS]=extract_eigenfunctions(U,V,OBS,DEPTH,l,r,R);
%
% inputs:
%   U, radial eigenfunctions
%   V, horizontal eigenfunctions
%   OBS, radial grid point that the observations are made ad
%   DEPTH, radial grid point where the source is located
%   l, angular degrees
%   r, grid in fractional radius
%   R, solar radius
% outputs:
%   UR, radial displacement eigenfunctions at the observation
%   radius
%   DS, divergence of the displacement eigenfunctions at the source
%   depth
%
%
% aaron birch, may 21, 2002

% allocate space for the divergence
[N,M]=size(U);
D=zeros(size(U));

% loop over the modes, computing the divergence 
for (i=1:N)
	D(i,:)=gradient(U(i,:),R*r)+(r*R).^(-1).*(2*U(i,:)-sqrt(l(i)*(l(i)+1))*V(i,:));
end;

%
% extract what we need
%
UR=U(:,OBS);
DS=D(:,DEPTH);