diff --git a/.github/workflows/pytest-unit-tests.yml b/.github/workflows/pytest-unit-tests.yml
index c0e3721..d1f1fd4 100644
--- a/.github/workflows/pytest-unit-tests.yml
+++ b/.github/workflows/pytest-unit-tests.yml
@@ -19,10 +19,10 @@ jobs:
     - name: Checkout simple_functions
       uses: actions/checkout@v2
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install dependencies
       run: |
diff --git a/simple_functions/__init__.py b/simple_functions/__init__.py
index 963db77..4399296 100644
--- a/simple_functions/__init__.py
+++ b/simple_functions/__init__.py
@@ -1,4 +1,5 @@
 from .functions1 import * # noqa
+from .constants import * # noqa
 
 from pkg_resources import get_distribution, DistributionNotFound
 try:
diff --git a/simple_functions/constants.py b/simple_functions/constants.py
new file mode 100644
index 0000000..b5c2b50
--- /dev/null
+++ b/simple_functions/constants.py
@@ -0,0 +1,15 @@
+from numpy import sqrt
+from simple_functions.functions1 import factorial
+from functools import cache
+
+__all__ = ['pi']
+
+
+def pi(terms=1):
+    return 1./(2.*sqrt(2.)/9801.*rsum(terms))
+
+
+@cache
+def rsum(n):
+    t = factorial(4*n)*(1103+26390*n)/(factorial(n)**4*396**(4*n))
+    return t + rsum(n-1) if n else t
diff --git a/simple_functions/functions1.py b/simple_functions/functions1.py
index 8c63a4d..49205fb 100644
--- a/simple_functions/functions1.py
+++ b/simple_functions/functions1.py
@@ -1,5 +1,7 @@
 
-__all__ = ['my_sum']
+from functools import cache
+
+__all__ = ['my_sum','factorial']
 
 
 def my_sum(iterable):
@@ -7,3 +9,8 @@ def my_sum(iterable):
     for i in iterable:
         tot += i
     return tot
+
+
+@cache
+def factorial(n):
+    return n * factorial(n-1) if n else 1
diff --git a/tests/test_constants.py b/tests/test_constants.py
new file mode 100644
index 0000000..1298113
--- /dev/null
+++ b/tests/test_constants.py
@@ -0,0 +1,13 @@
+import numpy as np
+
+from simple_functions import pi
+
+
+class TestPi(object):
+    '''Class to test our constants are computed correctly'''
+
+
+    def test_pi(self):
+        '''Test computation of pi'''
+        my_pi = pi(2)
+        assert np.isclose(my_pi, np.pi, atol=1e-12)
diff --git a/tests/test_simple_functions.py b/tests/test_simple_functions.py
index 5a03b52..ba6d498 100644
--- a/tests/test_simple_functions.py
+++ b/tests/test_simple_functions.py
@@ -1,6 +1,6 @@
 import pytest
 
-from simple_functions import my_sum
+from simple_functions import my_sum, factorial
 
 
 class TestSimpleFunctions(object):
@@ -14,3 +14,14 @@ def test_my_add(self, iterable, expected):
         '''Test our add function'''
         isum = my_sum(iterable)
         assert isum == expected
+
+
+    @pytest.mark.parametrize('number, expected', [
+        (5, 120),
+        (3, 6),
+        (1, 1)
+    ])
+    def test_factorial(self, number, expected):
+        '''Test our factorial function'''
+        answer = factorial(number)
+        assert answer == expected