diff --git a/README.md b/README.md
index 9ade200..a8720cf 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@ A Python package for graph kernels, graph edit distances and graph pre-image pro
 * tabulate>=0.8.2
 * tqdm>=4.26.0
 * control>=0.8.2 (for generalized random walk kernels only)
-* slycot==0.3.3 (for generalized random walk kernels only, which requires a fortran compiler, gfortran for example)
+* slycot>0.4.0 (for generalized random walk kernels only, which requires a fortran compiler, gfortran for example)
 
 ## How to use?
 
diff --git a/requirements.txt b/requirements.txt
index e67a6a3..2e8cbc0 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,6 +6,7 @@ scikit-learn>=0.20.0
 tabulate>=0.8.2
 tqdm>=4.26.0
 control>=0.8.2 # for generalized random walk kernels only.
+slycot>=0.4.0 # for generalized random walk kernels only, which requires a fortran compiler, gfortran for example
 cvxpy>=1.0.31 # for preimage. Does not work for "pip install graphkit-learn".
 # -e https://files.pythonhosted.org/packages/11/d0/d900870dc2d02ea74961b90c353666c6528a33ea61a10aa59a0d5574ae59/cvxpy-1.0.31.tar.gz # for preimage.
 cvxopt>=1.2.5 # for preimage.
diff --git a/requirements_pypi.txt b/requirements_pypi.txt
index d03fb10..1a42146 100644
--- a/requirements_pypi.txt
+++ b/requirements_pypi.txt
@@ -6,6 +6,7 @@ scikit-learn>=0.20.0
 tabulate>=0.8.2
 tqdm>=4.26.0
 control>=0.8.2 # for generalized random walk kernels only.
+# slycot>=0.4.0 # for generalized random walk kernels only, which requires a fortran compiler, gfortran for example
 # cvxpy>=1.0.31 # for preimage. Does not work for "pip install graphkit-learn".
 # -e https://files.pythonhosted.org/packages/11/d0/d900870dc2d02ea74961b90c353666c6528a33ea61a10aa59a0d5574ae59/cvxpy-1.0.31.tar.gz # for preimage.
 cvxopt>=1.2.5 # for preimage.