null: update documentation

1 files changed, 3 insertions, 1 deletions

diff --git a/null.py b/null.py
index 1bdc5ab..2ba59f0 100644
--- a/null.py
+++ b/null.py
@@ -27,7 +27,9 @@ def _null_geodesic_axis_rhs(Lambda, f,
 
     As p^μ should be null, it also holds
         0 = g_μν p^μ p^ν = g_tt (p^t)^2 + 2 g_tx p^t p^x + g_xx (p^x)^2
-     -> p^t = (-g_tx p^x ± √((g_tx p^x)^2 - g_tt g_xx (p^x)^2)) / g_tt
+     -> p^x = (-g_tx p^t ± √((g_tx p^t)^2 - g_tt g_xx (p^t)^2)) / g_xx
+    This constraint needs to be enforced at runtime to ensure the curve remains
+    null.
 
     With zero shift, it can be easily computed that
         Γ^t_tt =  0.5 g^tt g_tt,t