# Author's software

#### Being tested

Double factorial

Writing functions:

factorialdbl(x)

Unlike factorial, where the product of the numbers from 1 to the indicated number, then double factorial is a multiplication of numbers from 1 (or 2) to indicated number from step 2. The number x is always finite, for example:

factorialdbl(7)=1*3*5*7

factorialdbl(12)=2*4*6*10*12

A double factorial is a mathematical function that is used to calculate the product of all numbers starting from a certain number and considering only even or only odd numbers. It is indicated by a double exclamation mark and a number as an argument. For example, the double factorial of the number 6 is denoted as 6!!, and is calculated as 6 * 4 * 2 = 48.

The application of the double factorial can be useful in many fields, especially in combinatorics and probability theory. For example, it can be used to calculate the number of different combinations or permutations of elements in tasks related to the probability of selection from a set of objects.

Also, the double factorial can be useful when solving problems related to the number of ways to place objects in a sequence with certain rules or restrictions. Due to the ability to take into account only even or only odd numbers, the double factorial can be especially useful in such situations, allowing you to easily take into account certain properties or features inherent only to certain types of objects.

In addition, the double factorial can be used to model and calculate processes related to the evolution and change of objects or populations. For example, in biological and environmental research, it can be used to analyze changes in the abundance of a certain species or group of organisms over a certain period of time.

The main advantage of the double factorial is its ability to take into account only even or only odd numbers, which allows for more flexible and accurate modeling and analysis of various situations and processes. Due to this, the function is widely used in various fields of science and engineering.

In conclusion, the double factorial is a powerful tool for modeling, analyzing, and solving problems related to combinatorics, probability, population analysis, and other fields. Its use makes it possible to more accurately take into account the features and properties of objects, as well as effectively solve various tasks requiring analysis and calculations based on a combinatorial approach.

Help on built-in functions
The rules of programming scripts
Programm options
Color constants