How Square Roots Can Help You Check Prime Numbers

Tybie Fitzhugh, Henry Spencer, Mitch Harding, YbabFlow, Plenty W, Quinn Moyer, Julius 420, Philip Rogers, Ilmori Fajt, Brandon, August Taub, Ira Sanborn, Matthew Chudleigh, Cornelis Van Der Bent, Craig Butz, Mark S, Thorbjorn M H, Mathias Ermatinger, Edward Clarke, and Christopher Masto, Joshua S, Max, Joost Doesberg, Adam, Chris Reisenbichler, and Stan Seibert.

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This bonus video was filmed by Carlo Trappenberg.

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DISCLAIMER: any use of fire, tools, or other science experiments in this series is always done in a safe and professional way. Do not try to copy any actions you see in this series yourself.

[hkayakh]

I remember hearing numberphile say how you only need to check for divisors below the square root of a number but they never explained why. Now I finally have an explanation! Thank you!

[prathameshausekar]

I did not know that we need to check ONLY PRIMES less than the square root of number, that's great! Thank you so much.

[blnk]

I was trying to implement a program to check prime numbers and i found an example testing until the square root of the number and didnt figure it out why... You really helped me man, TY! Best of luck for your channel!

[schmorrell]

This man has no speech disfluencies. A pleasure to listen to.

[gewvdjq]

Finally someone who explains it the way all people can understand!! Thank you very much 🙏🙏

[klaouchie]

Wonderful, you're a very great teacher! (I am French and I fully understood what you said) also the technique is amazing!
Also I noticed that more the number between 1 and the root is close to the root, more the number between the root and the number that we want to know if it's primary will be close to the root either. (And more the number between 1 and the root is close to 1, more the number between the root and the number that we want to know if it's primary will be close to it)
Sorry my English is so bad 😅

[livedandletdie]

Well, there's an easy test for divisibility by 2, 3, 5, 7, 11...
A number is divisible by 2 if it ends with 0 or 2 or 4 or 6 or 8
A number is divisible by 3 if it's digit sum is divisible by 3
A number is divisible by 5 if it ends with 0 or 5
A number is divisible by 7 if you double the last digit subtract it from the other digits, and the result is divisible by 7. Example 105 10-10=0 and 0 is divisible by 7. Or 672 67-4=63 and 7 divides 63
A number is divisible by 11 if you add every other digit and subtract every other digit and the sum divides 11, so 1034 +1-0+3-4=0 0/divides 11

There are simple divisibility rules for most numbers.
You can easily construct these if you have somewhat an understanding of division.

[satyammishra]

Great explanation! Someone told me this concept but never knew the explanation.

[paulishism]

This is the exact video I was looking for.

Also you left out 19.

[conservatvepatrot]

His message rocks, intentional or not: Good information is good information, no matter where it comes from. You don’t need a sterile lab or classroom environment. You don’t need an influencer ring-light. You don’t need a fresh pressed lab coat and you don’t need perfectly quaffed hair. You can learn from *anyone* and we need to stop basing credibility on appearance.

To all y’all mention the “mess”; he knows what he’s doing.

[GetMeThere]

Thanks. I like you method of presentation. It gets into my head nicely. Thanks for posting.

[lylichavez]

Thank you for this helpful video! Your explanations are great

[renjugeorge]

Wonderful 👍🏼 thank you for this explanation

[foodfairyshi]

Thank you for the video, good and clear explanation!

[jayantrabindernath]

Thank you! This speeds up my algorithm output!

[likkcmg]

This helped me so much!!!! Thanks a ton !!!! :)

[shmug]

great explanation, always heard this but understand it finally

[paulvild]

Finally, a use for prime numbers, even if it's just to find out if a number is prime.
😂

[diribigal]

I think a couple times you said "divide" instead of "divide into", but context always made things clear. Nice video as always!

[lukandrate]

That's actually how I used to check for primes in python. I was just going through every odd number and 2 up to the floor(sqrt(x)) included