Phew, for a moment I worried that 2.9999... was divisible by 7 and I woke up in some kind of alternate universe
this post was submitted on 04 Nov 2024
679 points (98.0% liked)
Science Memes
11021 readers
3520 users here now
Welcome to c/science_memes @ Mander.xyz!
A place for majestic STEMLORD peacocking, as well as memes about the realities of working in a lab.
Rules
- Don't throw mud. Behave like an intellectual and remember the human.
- Keep it rooted (on topic).
- No spam.
- Infographics welcome, get schooled.
This is a science community. We use the Dawkins definition of meme.
Research Committee
Other Mander Communities
Science and Research
Biology and Life Sciences
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- !reptiles and [email protected]
Physical Sciences
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
- [email protected]
Humanities and Social Sciences
Practical and Applied Sciences
- !exercise-and [email protected]
- [email protected]
- !self [email protected]
- [email protected]
- [email protected]
- [email protected]
Memes
Miscellaneous
founded 2 years ago
MODERATORS
Can we just say it isn't? Like that's an exception, and then the rest of math can just go on like normal
So what? Being a prime number doesn't mean it can't be a divisor. Or is it the string of 9s that's supposed to be upsetting? Why? What difference does it make?
Thanks, Satan
Wait until you learn of 51/17.
Thanks, I hate it
With 17, I understand that you're referring to how 299,999 is also divisible by 17. What is the 51 reference, though? I know there's 3,999,999,999,999 but that starts with a 3. Not the same at all.
57 / 17 = 3. That messes up with my brain.
Your math is wrong.
That it is. And it's not just my math that is wrong.
⅐ = 0.1̅4̅2̅8̅5̅7̅
The above is 42857 * 7, but you also get interesting numbers for other subsets:
7 * 7 = 49
57 * 7 = 399
857 * 7 = 5999
2857 * 7 = 19999
42857 * 7 = 299999
142857 * 7 = 999999
Related to cyclic numbers:
142857 * 1 = 142857
142857 * 2 = 285714
142857 * 3 = 428571
142857 * 4 = 571428
142857 * 5 = 714285
142857 * 6 = 857142
142857 * 7 = 999999
I know you opened your calculator app to check it.
49 is divisible by 7, so why not?
…9999 is exactly equal to -1.
But only in base 10.
Well yes of course. If it was a different base, writing it that way if the symbol was even available would be a different number.
Never realized there are so many rules for divisibility. This post fits in this category:
Forming an alternating sum of blocks of three from right to left gives a multiple of 7
299,999 would be 999 - 299 = 700 which is divisible by 7. And if we simply swap grouped digits to 999,299, it is also divisible by 7 since 299 - 999 = -700.
And as for 13:
Form the alternating sum of blocks of three from right to left. The result must be divisible by 13
So we have 999 - 999 + 299 = 299.
You can continue with other rules so we can then take this
Add 4 times the last digit to the rest. The result must be divisible by 13.
So for 299 it's 29 + 9 * 4 = 65 which divides by 13. Pretty cool.
That is indeed an absurd amount of rules (specially for 7) !
It should be fun to develop each proof. Particularly the 1,3,2,-1,-3,-2 rule, which at first sight seems could be easily expanded to any other number.
The divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7.
E.g. 299'999 → 29'999 - 18 = 29'981 → 2'998 - 2 = 2'996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0.
It's a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.
I think it might be easier just to do the division.
Thanks I hate it
This math will not stand man!
If all of the digits summed recursively reduce to a 9, then the number is divisible by 9 and also by 3.
If the difference between the sums of alternating sets digits in a number is divisible by 11, then the number itself is divisible by 11.
That’s all I can remember, but yay for math right?
The 9 rule works for 3 too The 6 rule is if (divisible by 3 and divisible by 2)
Well, on the side of easy ones there is "if the last digit is divisible by 2, whole number is divisible by 2". Also works for 5. And if you take last 2 digits, it works for 4. And the legendary "if it ends with 0, it's divisible by 10".
Its never divisible by zero, and its always divisible by one
There’s also the classic “no three positive integers a, b, and c to satisfy a**n + b**n = c**n for values of n greater than 2“ trick but my proof is too large to fit in this comment.
Fucking lol
Isn’t every number divisible by 7?
Yes technically almost every number is divisible by another in some way and you're left with a remainder that spans plenty of decimal places.
But common parlance when something is said to be divisible is that the end results is a round number...
Yup, this is just a coordinated smear campaign from Big Integer.
Actually disgusting
Why
42857 for those who wonder
And for ops title: 23076923