• Title: Factorial

• Bright video: https://youtu.be/Y4yZaEuoThs

• Dark video: https://youtu.be/rogCR0ZJrTU

• Timestamps
• Subtitle in English

1 00:00:00,500 –> 00:00:03,788 The mathematical symbol of today is the factorial.

2 00:00:03,988 –> 00:00:06,059 Which is written with an exclamation mark.

3 00:00:07,157 –> 00:00:10,136 and usually you see the definition for a natural number n.

4 00:00:11,357 –> 00:00:18,243 This is because this n with an exclamation mark, represents a product with exactly n factors.

5 00:00:18,929 –> 00:00:23,626 We start with the number n itself. Then we reduce the number by 1

6 00:00:24,457 –> 00:00:29,016 and then we continue this whole procedure until we reach the number 1.

7 00:00:29,929 –> 00:00:36,762 This means when you see this definition, you know we need a positive integer n such that this makes sense.

8 00:00:37,714 –> 00:00:40,471 For example when you see 4!

9 00:00:40,571 –> 00:00:45,343 This just means you have 4 times 3 times 2 times 1.

10 00:00:46,214 –> 00:00:49,068 Which is, as one can remember 24.

11 00:00:50,486 –> 00:00:54,770 also you should remember 1! is just 1.

12 00:00:55,614 –> 00:01:01,282 Now what you see here with the factorial is what in mathematics we call a recursive definition.

13 00:01:02,000 –> 00:01:07,228 This means that we first fix a starting value and indeed we can do this with 0.

14 00:01:08,200 –> 00:01:11,287 Here 0! should be defined as 1.

15 00:01:12,500 –> 00:01:17,443 This makes sense for a lot of formulas, so this is indeed something you really should remember.

16 00:01:18,343 –> 00:01:24,977 and then the next step in the recursive definition is saying what happens with the successor of a given natural number.

17 00:01:25,714 –> 00:01:30,284 So n! should be n times (n-1)!

18 00:01:30,943 –> 00:01:36,378 So when we know what the factorial does to (n-1), we also know what it does to n.

19 00:01:37,157 –> 00:01:43,212 and because we have a starting value, we now could calculate the values for all other natural numbers n.

20 00:01:44,229 –> 00:01:50,069 Ok, this is the definition for the factorial. Which you can apply for natural numbers n.

21 00:01:50,886 –> 00:01:53,468 Thanks for listening and see you next time.

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