diff --git a/nanochat/optim.py b/nanochat/optim.py
index 4cc2a1f..baf95ad 100644
--- a/nanochat/optim.py
+++ b/nanochat/optim.py
@@ -247,6 +247,7 @@ class MuonAdamW(torch.optim.Optimizer):
         momentum_buffer = state["momentum_buffer"]
 
         # Second momentum buffer is factored, either per-row or per-column
+        # from NorMuon: https://arxiv.org/abs/2510.05491
         if "second_momentum_buffer" not in state:
             state_shape = (num_params, shape[-2], 1) if shape[-2] >= shape[-1] else (num_params, 1, shape[-1])
             state["second_momentum_buffer"] = torch.zeros(state_shape, dtype=dtype, device=device)