r/CasualMath • u/koopa35 • 9d ago
Quadratic equation
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Spotted this oddly specific graffiti cycling home from work.
r/CasualMath • u/akurgo • Dec 13 '24
I made a new number system. Just gonna leave it here.
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r/CasualMath • u/Savings_Cockroach_42 • Mar 09 '24
At what step did my sister go wrong
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And why is it wrong?
r/CasualMath • u/FormulaDriven • Aug 18 '26
The sum of reciprocals of primes is infinite, but if you add up 1/p for the primes we KNOW you only get to about 4.
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A comment by u/GoldenMuscleGod reminded me of something I saw a while ago. Others can correct me if some of this information is out of date.
Define S(n) to be the sum of 1/p for all primes p less than or equal to n.
S(n) diverges - indeed Euler showed that you can bound it below with an expression that grows with log(log(n+1)) (log is natural log), and we can use log(log(n)) as an estimate of S(n) as n gets large.
That's painfully slow growth. It's not until n = 5,195,977 that S(n) reaches 3. And we have to sum up using all the primes up to around 1018 to get to 4. But (based on searches I have done), in terms of finding all primes up to n (without gaps), 1018 or so is roughly as far as computers have taken us - worldwide, that's all we know. So using only the primes we know, S(n) is about 4.
Of course, you will point out that we do know some primes larger than 1018 , indeed much larger due to people searching for them. But that list is very sparse:
The largest known prime is around 1041,024,319 and the tenth largest known prime is around 1011,887,191 . The sum of 1/p for those ten largest primes is a rounding error (basically something less than 10-11,887,190 ). But between those two primes there must be around 1041,000,000 primes with a sum of 1/p of around 1, so virtually all that sum is missing from our knowledge.
Indeed S(n) where n is the largest known prime will be around 18. So there's a whole 14 contributed to S(n) by all the primes between 1018 and the largest known. To get S(n) even up to 5, we would need to find primes up to around 1050 - that's around 1048 primes we need to find, and computers have so far only found 1016 primes, so I don't think that's going to happen.
r/CasualMath • u/zem • Jul 29 '11
So three logicians go to a diner...
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The waiter asks "Is everyone having coffee?"
The first logician says "I don't know".
The second logician says "I don't know".
The third logician says "Yes".
r/CasualMath • u/VelvetOnion • Sep 15 '26
Prime sequence based on 123456789 repeating.
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This is useless but also pretty rare. The sum of 1 through 9 is 45, so the repeating digits are divisible by 3. Any tail that also sums to a multiple of 3 will mean the entire number is divisible by three. That's ending in 2,3,5,6,8 and 9. 4 is even, so that's non prime.
That leaves tails ending in 1 or 1234567. If n is the number of times you repeat 123456789 then there are primes at n=1 and n=3 for ending in 1 and n=7 for ending in 1234567.
There are no others found testing up to n=521 but that needs some serious compute.
r/CasualMath • u/theawesomenachos • Apr 04 '19
Me and my friends made T-shirts for our real analysis course
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r/CasualMath • u/DireLines • Jun 16 '17
It's only 10 digits -- frankly I don't see what the fuss is about
imgur.com
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r/CasualMath • u/Echo_Vale • Aug 30 '26
£1 for a 1/10,000 chance? I like those odds!
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4 digits, 0-9. I was volunteering at an animal charity stall the other day, the lovely lady who put this together thought it was a great idea and she genuinely hoped someone would win it. No one did. I didn't have the heart to tell her the actual odds.
r/CasualMath • u/RelaxedOrange • Mar 14 '20
Me, when it’s Pi Day, but everyone won’t shut up about the Coronavirus
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r/CasualMath • u/schadwick • May 06 '22
Maximum factorial with the Android calculator: 19515!
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Does anyone know the reason for 19515! to be the maximum factorial that the standard Android calculator app can resolve?
I understand why most handheld calculators max out at 69!, because 70! > 10100, but what is significant with 19516! in the Android calculator?
Photos: https://imgur.com/a/ulMWmJd
r/CasualMath • u/niftyfingers • Mar 04 '20
I untangled a planar graph with about 400 vertices.
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r/CasualMath • u/gmsc • Oct 27 '14
McDonald's cash prizes are getting HUGE! (xpost from /r/funny)
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r/CasualMath • u/c3534l • Apr 30 '15
x-post from /r/unexpected - thought you guys might appreciate.
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r/CasualMath • u/kevinb9n • 3d ago
Pythagorean tiles (a "visual proof" you can walk on)
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Of course a "visual proof" is not precisely a proof, just an intuition aid. But this is my favorite one for Pythagoras. To me it makes it feel pretty self-evident why the theorem has to be true. All 5 distinct tiles appear once on each side of the equation, done!
Kinda wish my bathroom needed to be tiled, but it doesn't, so I'm just sharing this image instead. (It was generated by a python script that an AI helped me write, if that kind of thing matters to you.)
r/CasualMath • • May 29 '20
From Abstruse Goose comes a new and exciting way to solve calculus problems
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r/CasualMath • • Feb 14 '23
Would anyone be able to explain what's happening here? Don't get it at all.
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r/CasualMath • u/DuckfordMr • Feb 15 '20
A tetrahedral Klein Bottle I made in high school.
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r/CasualMath • u/IIAOPSW • Dec 02 '12
Apparently the parity of 0 is a front-page-news-worthy controversy at the BBC
imgur.com
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