
Title: Euler’s Phi Function

Series: Advent of Mathematical Symbols

YouTubeTitle: Advent of Mathematical Symbols  Part 19  Euler’s Phi Function

Bright video: https://youtu.be/XDnksOa3GMk

Dark video: https://youtu.be/9vLKYMmQhGc

PrintPDF: Download printable PDF version

Thumbnail (bright): Download PNG

Timestamps

Subtitle in English
1 00:00:00,629 –> 00:00:02,139 Hello and welcome.
2 00:00:02,371 –> 00:00:05,957 and the mathematical symbol of today is Euler’s phi function.
3 00:00:06,129 –> 00:00:08,251 Denoted with a lower case phi.
4 00:00:09,257 –> 00:00:14,057 It’s also called Euler’s totient function and used in number theory.
5 00:00:14,943 –> 00:00:19,605 Therefore the domain of the function is given by the positive integers.
6 00:00:20,300 –> 00:00:24,006 So we have the natural numbers N. Starting with 1.
7 00:00:24,943 –> 00:00:28,189 and also the codomain is given by the natural numbers.
8 00:00:29,157 –> 00:00:33,989 Now of course the question here should be: What is the definition of phi of n?
9 00:00:34,614 –> 00:00:40,061 and the short answer is that we simply have to count numbers with 2 properties.
10 00:00:40,886 –> 00:00:45,959 So lets use the letter “a” for the numbers we count and then i explain the 2 properties.
11 00:00:46,743 –> 00:00:51,900 The first one is very simple. We only consider numbers that are less or equal than n.
12 00:00:52,600 –> 00:00:57,734 This means that the outcome of the phi function can never be greater than the input.
13 00:00:58,600 –> 00:01:04,574 Then the second property tells us that “a” and n don’t have a single common divisor.
14 00:01:05,357 –> 00:01:08,947 Of course except 1, because 1 divides everything.
15 00:01:09,857 –> 00:01:14,043 Hence we can write this as the greatest common divisor.
16 00:01:14,629 –> 00:01:17,284 gcd of “a” and n.
17 00:01:18,343 –> 00:01:20,157 Is equal to 1.
18 00:01:21,229 –> 00:01:25,585 In mathematics then we say that “a” and n are mutually prime.
19 00:01:26,743 –> 00:01:32,417 Now if you have never seen this greatest common divisor then i think examples will help.
20 00:01:33,014 –> 00:01:35,773 For example what is phi of 4?
21 00:01:36,557 –> 00:01:42,918 Now, because of property 1 we already know we only have to look at the numbers 1, 2, 3 and 4.
22 00:01:43,543 –> 00:01:49,853 Of course property 2 immediately tells us that the number itself so 4 here can not be counted.
23 00:01:50,671 –> 00:01:56,942 Indeed you see the gcd(4, 4) would be 4 again. Not 1.
24 00:01:57,886 –> 00:02:02,143 However then you also see that 1 does not have any divisors.
25 00:02:02,228 –> 00:02:05,251 Therefore it always fulfils both properties.
26 00:02:06,200 –> 00:02:10,894 Then in the next step we have to exclude 2, because 2 times 2 is 4.
27 00:02:12,129 –> 00:02:17,604 Ok then looking at 3 we see there is no common divisor greater than 1.
28 00:02:18,257 –> 00:02:24,466 and now you see when we count the numbers that remain that fulfil both properties we get out 2.
29 00:02:25,157 –> 00:02:27,731 Hence phi of 4 is 2
30 00:02:28,443 –> 00:02:32,079 So maybe lets look at another example. So lets choose 5.
31 00:02:33,071 –> 00:02:38,273 Now we can look at the list again, but maybe you already know that 5 is a prime number.
32 00:02:39,143 –> 00:02:43,031 Hence the only number in the list we can cancel would be 5.
33 00:02:44,271 –> 00:02:47,355 Therefore finally we count 4 numbers.
34 00:02:48,500 –> 00:02:55,515 In fact with this reasoning here we can immediately calculate phi of p. When p is a prime number.
35 00:02:56,614 –> 00:02:59,977 and now you know this should be (p  1).
36 00:03:00,843 –> 00:03:07,702 Now there a lot of other nice properties for Euler’s phi function, but maybe this is good enough for this video.
37 00:03:08,629 –> 00:03:11,084 Therefore i hope that i see you next time.
38 00:03:11,284 –> 00:03:12,157 Bye!