Euler's totient function | Journey into cryptography | Computer Science | Khan Academy

Wow, crazy how hard it was to find a solid explanation that was easy to grasp. Not to mention in little over 2 minutes! Well done.

You just helped explain an entire cryptography chapter in 2 minutes. Thank you!

So solve Fermat's little theorem faster ?

Have watched this video several times and I always enjoy it

May I know why 1 isn't counted? Like 1 and 8 shares the common factor of 1 isn't it?

:54 "Look at this graph"

good

background music was creepy as hell

So totient of a product of 2 different primes is always divisible by 4

Keep in mind phi is multiplicative only when both factors are coprime. Otherwise a very clean explanation.

But why is that the phi(121) = 110? Shouldn't it be 100 since phi(11*11) = 10 * 10???

thumbs down for incorrect statement. phi function is only multiplicative if the two numbers are coprime

Could you please help? How do we find n, from fi(n)=20, for example? I mean by pen and paper.

What about phi(25)?
phi(25)=phi(5*5). Since 5 is prime, we should be able to say phi(25)=phi(5*5)=(5-1)(5-1)=16, but phi(25)=20.

Phi is only multiplicative if the two numbers are prime. phi(20*34) doesn't equal phi(20)*phi(34).