diff --git a/imgx/diffusion/gaussian/variance_schedule.py b/imgx/diffusion/gaussian/variance_schedule.py
index 1f37b72..3a933b7 100644
--- a/imgx/diffusion/gaussian/variance_schedule.py
+++ b/imgx/diffusion/gaussian/variance_schedule.py
@@ -107,6 +107,8 @@ def downsample_beta_schedule(
 
     After down-sampling, the first and last values of alphas_cumprod are kept.
 
+    https://arxiv.org/abs/2102.09672
+
     Args:
         betas: beta schedule, shape (num_timesteps,).
             Values are in ascending order.