Hexagonal number sequence (#5640)

* Hexagonal number sequence 

A hexagonal number sequence is a sequence of figurate numbers where the nth hexagonal number hₙ is the number of distinct dots in a pattern of dots consisting of the outlines of regular hexagons with sides up to n dots, when the hexagons are overlaid so that they share one vertex.

This program returns the hexagonal number sequence of n length.

* Update hexagonalnumbers.py

* Update hexagonalnumbers.py

* Update hexagonalnumbers.py

* Update hexagonalnumbers.py

* Update and rename hexagonalnumbers.py to hexagonal_numbers.py

* Length must be a positive integer
Co-authored-by: NChristian Clauss <cclauss@me.com>

maths/series/hexagonal_numbers.py

+"""
+A hexagonal number sequence is a sequence of figurate numbers
+where the nth hexagonal number hₙ is the number of distinct dots
+in a pattern of dots consisting of the outlines of regular
+hexagons with sides up to n dots, when the hexagons are overlaid
+so that they share one vertex.
+
+    Calculates the hexagonal numbers sequence with a formula
+        hₙ = n(2n-1)
+        where:
+        hₙ --> is nth element of the sequence
+        n --> is the number of element in the sequence
+        reference-->"Hexagonal number" Wikipedia
+        <https://en.wikipedia.org/wiki/Hexagonal_number>
+"""
+
+
+def hexagonal_numbers(length: int) -> list[int]:
+    """
+    :param len: max number of elements
+    :type len: int
+    :return: Hexagonal numbers as a list
+
+    Tests:
+    >>> hexagonal_numbers(10)
+    [0, 1, 6, 15, 28, 45, 66, 91, 120, 153]
+    >>> hexagonal_numbers(5)
+    [0, 1, 6, 15, 28]
+    >>> hexagonal_numbers(0)
+    Traceback (most recent call last):
+      ...
+    ValueError: Length must be a positive integer.
+    """
+
+    if length <= 0 or not isinstance(length, int):
+        raise ValueError("Length must be a positive integer.")
+    return [n * (2 * n - 1) for n in range(length)]
+
+
+if __name__ == "__main__":
+    print(hexagonal_numbers(length=5))
+    print(hexagonal_numbers(length=10))