for xi = 1:251
	x = (xi-10)/50;
	for yi = 1:71
		y = (yi-10)/50;
		xm(xi, yi) = x;
		ym(xi, yi) = y;
		if(y < 0)
			pxy(xi,yi) = 0;
		else 
			if(y > 1)
				pxy(xi,yi) = 0;
			else 
				if(x >= y)
					pxy(xi,yi) = exp(-(x-y));
				else
					pxy(xi,yi) = 0;
				end
			end
		end
	end
end

surf(xm, ym, pxy);
shading('interp');