diff --git a/examples/heisenberg_tDMRG/heisenberg_groundstate.ipynb b/examples/heisenberg_tDMRG/heisenberg_groundstate.ipynb
index 3b21d9a12d3095f9fe09f6fd404feba112dc2a1d..6a1d2053078471937b19debca8156f371fce180f 100644
--- a/examples/heisenberg_tDMRG/heisenberg_groundstate.ipynb
+++ b/examples/heisenberg_tDMRG/heisenberg_groundstate.ipynb
@@ -1,5 +1,12 @@
 {
  "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Algorithm after http://arxiv.org/pdf/1008.3477v2.pdf chapter 7.3.1"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 1,