function [r2] =  radius2(xi, l, e, M)
% Schwarzschild spacelike geodesic
% Radius in Schwarzschild space-time
% for spacelike geodesic of the second kind
% l and e are parameters
% xi is a variable.
r2 = 1./(1/2/M-2/l +(1/2/M-(3+e)/l)*tan(xi/2).^2);