diff --git a/DIRECTORY.md b/DIRECTORY.md
index 9e0166ad8..c37c4f99b 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -541,7 +541,7 @@
   * [Dimensionality Reduction](machine_learning/dimensionality_reduction.py)
   * Forecasting
     * [Run](machine_learning/forecasting/run.py)
-  * [Frequent Pattern Growth Algorithm](machine_learning/frequent_pattern_growth.py)
+  * [Frequent Pattern Growth](machine_learning/frequent_pattern_growth.py)
   * [Gradient Descent](machine_learning/gradient_descent.py)
   * [K Means Clust](machine_learning/k_means_clust.py)
   * [K Nearest Neighbours](machine_learning/k_nearest_neighbours.py)
@@ -649,6 +649,7 @@
   * [Numerical Integration](maths/numerical_integration.py)
   * [Odd Sieve](maths/odd_sieve.py)
   * [Perfect Cube](maths/perfect_cube.py)
+  * [Perfect Number](maths/perfect_number.py)
   * [Perfect Square](maths/perfect_square.py)
   * [Persistence](maths/persistence.py)
   * [Pi Generator](maths/pi_generator.py)
@@ -767,7 +768,6 @@
     * [Swish](neural_network/activation_functions/swish.py)
   * [Back Propagation Neural Network](neural_network/back_propagation_neural_network.py)
   * [Convolution Neural Network](neural_network/convolution_neural_network.py)
-  * [Perceptron](neural_network/perceptron.py)
   * [Simple Neural Network](neural_network/simple_neural_network.py)
 
 ## Other
@@ -803,8 +803,10 @@
   * [Archimedes Principle Of Buoyant Force](physics/archimedes_principle_of_buoyant_force.py)
   * [Basic Orbital Capture](physics/basic_orbital_capture.py)
   * [Casimir Effect](physics/casimir_effect.py)
+  * [Center Of Mass](physics/center_of_mass.py)
   * [Centripetal Force](physics/centripetal_force.py)
   * [Coulombs Law](physics/coulombs_law.py)
+  * [Doppler Frequency](physics/doppler_frequency.py)
   * [Grahams Law](physics/grahams_law.py)
   * [Horizontal Projectile Motion](physics/horizontal_projectile_motion.py)
   * [Hubble Parameter](physics/hubble_parameter.py)
diff --git a/boolean_algebra/nor_gate.py b/boolean_algebra/nor_gate.py
index 2c27b80af..0c8ab1c0a 100644
--- a/boolean_algebra/nor_gate.py
+++ b/boolean_algebra/nor_gate.py
@@ -1,15 +1,18 @@
 """
-A NOR Gate is a logic gate in boolean algebra which results to false(0)
-if any of the input is 1, and True(1) if  both the inputs are 0.
+A NOR Gate is a logic gate in boolean algebra which results in false(0) if any of the
+inputs is 1, and True(1) if all inputs are 0.
 Following is the truth table of a NOR Gate:
-   | Input 1 | Input 2 |  Output |
-   |      0      |     0      |      1      |
-   |      0      |     1      |      0      |
-   |      1      |     0      |      0      |
-   |      1      |     1      |      0      |
+    Truth Table of NOR Gate:
+    | Input 1  | Input 2  |  Output  |
+    |    0     |    0     |    1     |
+    |    0     |    1     |    0     |
+    |    1     |    0     |    0     |
+    |    1     |    1     |    0     |
 
-Following is the code implementation of the NOR Gate
+    Code provided by Akshaj Vishwanathan
+https://www.geeksforgeeks.org/logic-gates-in-python
 """
+from collections.abc import Callable
 
 
 def nor_gate(input_1: int, input_2: int) -> int:
@@ -30,19 +33,35 @@ def nor_gate(input_1: int, input_2: int) -> int:
     return int(input_1 == input_2 == 0)
 
 
-def main() -> None:
-    print("Truth Table of NOR Gate:")
-    print("|   Input 1   |  Input 2  | Output |")
-    print(f"|      0      |    0      |  {nor_gate(0, 0)}     |")
-    print(f"|      0      |    1      |  {nor_gate(0, 1)}     |")
-    print(f"|      1      |    0      |  {nor_gate(1, 0)}     |")
-    print(f"|      1      |    1      |  {nor_gate(1, 1)}     |")
+def truth_table(func: Callable) -> str:
+    """
+    >>> print(truth_table(nor_gate))
+    Truth Table of NOR Gate:
+    | Input 1  | Input 2  |  Output  |
+    |    0     |    0     |    1     |
+    |    0     |    1     |    0     |
+    |    1     |    0     |    0     |
+    |    1     |    1     |    0     |
+    """
+
+    def make_table_row(items: list | tuple) -> str:
+        """
+        >>> make_table_row(("One", "Two", "Three"))
+        '|   One    |   Two    |  Three   |'
+        """
+        return f"| {' | '.join(f'{item:^8}' for item in items)} |"
+
+    return "\n".join(
+        (
+            "Truth Table of NOR Gate:",
+            make_table_row(("Input 1", "Input 2", "Output")),
+            *[make_table_row((i, j, func(i, j))) for i in (0, 1) for j in (0, 1)],
+        )
+    )
 
 
 if __name__ == "__main__":
     import doctest
 
     doctest.testmod()
-    main()
-"""Code provided by Akshaj Vishwanathan"""
-"""Reference: https://www.geeksforgeeks.org/logic-gates-in-python/"""
+    print(truth_table(nor_gate))