random.randint(a, b) includes b, where b=4000 would cause exception (#425)

* random.randint(a, b) includes b, where b=4000 would cause exception

* math.factorial only works with integers starting python 3.9

* A constant equation does not have a stationary point

mathgenerator/calculus.py

@@ -82,7 +82,12 @@ def stationary_points(max_exp=3, max_coef=10):
     | $f(x)=6x^3 + 6x^2 + x + 8$ | ${- \frac{1}{3} - \frac{\sqrt{2}}{6}, - \frac{1}{3} + \frac{\sqrt{2}}{6}}$ |
     """
     solution = ''
-    while len(solution) == 0:
+
+    # A constant function has no stationary points, and the answer will be Reals. e.g.: 
+    #   x = sympy.symbols('x')
+    #   s = sympy.stationary_points(1 + 0 * x , x)
+    #   assert s == sympy.Reals
+    while solution == sympy.Reals or len(solution) == 0:
         x = sympy.symbols('x')
         problem = 0
         for exp in range(max_exp + 1):

mathgenerator/misc.py

@@ -97,6 +97,9 @@ def base_conversion(max_num=60000, max_base=16):
 
 def _newton_symbol(n, k):
     """Utility of binomial_distribution()"""
+    # math.factorial only works with integers starting python 3.9
+    # ref: https://docs.python.org/3/library/math.html#math.factorial
+    n, k = int(n), int(k)   
     return math.factorial(n) / (math.factorial(k) * math.factorial(n - k))
 
 
@@ -190,13 +193,14 @@ def complex_to_polar(min_real_imaginary_num=-20, max_real_imaginary_num=20):
     return problem, f'${theta}$'
 
 
-def decimal_to_roman_numerals(max_decimal=4000):
+def decimal_to_roman_numerals(max_decimal=3999):
     """Decimal to Roman Numerals
 
     | Ex. Problem | Ex. Solution |
     | --- | --- |
     | The number $92$ in roman numerals is:  | $XCII$ |
     """
+    assert 0 <= max_decimal <= 3999, f"max_decimal({max_decimal}) must be <= 3999"
     x = random.randint(0, max_decimal)
     x_copy = x
     roman_dict = {