Understanding Factorials and Multiplying by an Integer

In summary, factorials are denoted by an exclamation mark (!) and are used to multiply all the positive integers from 1 to n. When multiplying n by an integer before taking the factorial, the sequence starts at that integer and continues up to 2n, with the sequence being split into two parts (1 to n and n+1 to 2n) and then multiplied together.
  • #1
sandy.bridge
798
1

Homework Statement


Hey all. Not super familiar with using factorials, however, they do pop up occasionally. I understand that n! = 1*2*3*...*n. How do we treat factorial when we are multiplying n by an integer before taking the factorial? I know the answer for expanding (2n)!, however, I do not see why. Thanks in advance.
 
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  • #2
What don't you see? Think of this:

4! = 4*3*2*1

(2*4)! = 8! = 8*7*6*5*4*3*2*1
 
  • #3
so (2n)!=(2*1)*(4*3*2*1)*(6*5*4*3*2*1)*...*(2n*(2n-1)*...*1). This can be taken a step further, though correct?
 
  • #4
sandy.bridge said:
so (2n)!=(2*1)*(4*3*2*1)*(6*5*4*3*2*1)*...*(2n*(2n-1)*...*1). This can be taken a step further, though correct?

I don't think you get it. Look at post #2 again.
 
  • #5
(2n)!=(2n*(2n-1)*(2n-2)*...*1) ?
 
  • #6
sandy.bridge said:
(2n)!=(2n*(2n-1)*(2n-2)*...*1) ?
That's more like it!
 
  • #7
Okay. How exactly does that end up being: 1*2*3*...*n*(n+1)*(n+2)*(n+3)*...*(2n) ?
 
  • #8
sandy.bridge said:
Okay. How exactly does that end up being: 1*2*3*...*n*(n+1)*(n+2)*(n+3)*...*(2n) ?

How did (2*4)! = 8! end up being 1*2*3* ... *8? What are you not seeing?
 
  • #9
Look at counting to 2n this way:

1,2,3,4,...,n-2,n-1,n - the sequence of all integers from 1 to n
n+1,n+2,n+3,..., n+n-2,n+n-1,n+n - the sequence of integers from n+1 to 2n
 
  • #10
sandy.bridge said:
(2n)!=(2n*(2n-1)*(2n-2)*...*1) ?
sandy.bridge said:
Okay. How exactly does that end up being: 1*2*3*...*n*(n+1)*(n+2)*(n+3)*...*(2n) ?

You've written it down correctly twice - one of them is in reverse order of the other. How can you not know that they are the same?

1*2 = 2*1 etc
 
  • #11
SteamKing said:
Look at counting to 2n this way:

1,2,3,4,...,n-2,n-1,n - the sequence of all integers from 1 to n
n+1,n+2,n+3,..., n+n-2,n+n-1,n+n - the sequence of integers from n+1 to 2n
For some reason I was having a hard time seeing from n+1 to 2n. I completely see it now. Thanks!
 

1. What is a factorial?

A factorial is a mathematical operation denoted by an exclamation mark (!) that represents the product of all positive integers from 1 up to a given number. For example, 4! (read as "four factorial") is equal to 1 x 2 x 3 x 4 = 24.

2. How do you calculate a factorial?

To calculate a factorial, you simply multiply all positive integers from 1 up to the given number. For example, to calculate 5!, you would do 1 x 2 x 3 x 4 x 5 = 120. You can also use a calculator or a factorial function in programming languages to calculate factorials.

3. What is the purpose of multiplying by an integer in factorials?

Multiplying by an integer in factorials allows us to find the factorial of numbers that are not necessarily integers. For example, if we want to find 4.5!, we can multiply 4.5 by 3.5 (since 3.5 is the integer value before 4.5) to get 15.75. This allows us to extend the concept of factorials to non-integer numbers.

4. Can you multiply negative numbers in factorials?

No, factorials are only defined for positive integers. Multiplying negative numbers in factorials would not make mathematical sense and would result in an undefined answer.

5. What is the largest factorial that can be calculated?

The largest factorial that can be calculated depends on the limitations of the computer or calculator being used. However, as the factorial operation involves multiplying increasingly larger numbers, the calculated value can quickly become too large to be represented accurately in most systems.

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