function x = dsum(x,k)%DSUM%	Differential sum function.  If X is a vector [x(1) x(2) ... x(n)],%	then DSUM(X) returns a vector of sums between adjacent%	elements [x(2)+x(1)  x(3)+x(2) ... x(n)+x(n-1)].  If X is a%	matrix, the sums are calculated down each column:%	DSUM(X) = X(2:n,:) + X(1:n-1,:).%	DSUM(X,n) is the n'th differential sum function.%	Michael Maurer, 22 March 1991, after DIFF.M by J.N. Little 8-30-85if nargin == 1	k = 1;endfor i=1:k	[m,n] = size(x);	if m == 1		x = x(2:n) + x(1:n-1);	else		x = x(2:m,:) + x(1:m-1,:);	endend