diff --git a/proto/FTCS.ipynb b/proto/FTCS.ipynb
index f78119e..cac5bce 100644
--- a/proto/FTCS.ipynb
+++ b/proto/FTCS.ipynb
@@ -134,7 +134,7 @@
     "        # top\n",
     "        for j in range(0, grid_size['x']):\n",
     "               C_t1[0,j] = C_t[0,j] \\\n",
-    "                        +  max_stable_time_step/delta_y**2 * (alpha_interblock(alpha_y[1,j], alpha_x[0,j]) * C_t[1,j] \n",
+    "                        +  max_stable_time_step/delta_y**2 * (alpha_interblock(alpha_y[1,j], alpha_y[0,j]) * C_t[1,j] \n",
     "                                - (alpha_interblock(alpha_y[1,j], alpha_y[0,j]) + 2 * alpha_y[0,j]) * C_t[0,j]\n",
     "                                + 2 * alpha_y[0,j] * bc_top)\n",
     "\n",