Amazon is running a Prime Day sale on July 16 and 17. Setting aside the fact that this is two separate days, neither 716 nor 717 are prime numbers. They should’ve done 7/19 instead.
167 is prime though?
But 197 satisfies all
Please don’t ascribe any more meaning to prime day other than a cynical late-stage-capitalist plot for money to flow from the masses to bezos.
No, I’m taking back the word “prime” from a company that shouldn’t have exclusive rights to define the term. I’m not going to cede that territory just because I don’t like the company.
seems like the opposite, I’d suggest to stop reminding people that amazon prime day is a thing
July 16th is the 197th day of the year on non leap years. July 17th is the 199th day of the year on leap years.
Both of those are prime.
What about people using the normal date system
ISO 8601 is the only normal date system.
How the hell is 717 not a prime number? Who fucked that up? I vote we just change that
Divisible by 3. Easy to check since 7 + 1 + 7 = 15 which is divisible by 3.
Oh awesome that’s a neat trick I’ve never seen before. How does that work? For a number like 700 for example, 7 + 0 + 0 = 7 but 700 is visible by 10.
You can only use this method to check if the number can be divided by 3.
It works for 9, too.
If you’re looking for a proof:
Our base 10 system represents numbers by having little multipliers in front of each power of 10. So a number like 1234 is 1 x 10^3 + 2 x 10^2 + 3 x 10^1 + 4 x 10^0 .
Note that 10 is just (3 x 3) + 1. So for any 2 digit number, you’re looking at the first digit times (9 + 1), plus the second digit. Or:
(9 times the first digit) + (the first digit) + (the second digit).
Well we know that 9 times the first digit is definitely divisible by both 3 and 9. And we know that adding two divisible-by-n numbers is also divisible by n.
So we can ignore that first term (9 x first digit), and just look to whether first digit plus second digit is divisible. If it is, then you know that the original big number is divisible.
And when you extend this concept out to 3, 4, or more digit numbers, you see that it holds for every power of 10, and thus, every possible length of number. For both 9 and 3.
Back in the olden days before digital calculators, people had tricks like that to help them do mental math.
😂 I’m familiar with many of them because I liked math in school and went through all of primary and middle school without a digital calculator. This one is one I hadn’t heard of before and, again because I like math, I’m interested in understanding 1) what the trick is and 2) mathematically how it works out
How often do prime numbers occur in epoch time?
Well the convention was to store it as a 32 bit signed integer, so that is any number from -2^31 to (2^31 - 1). Prime numbers are formally defined as a subset of whole numbers, so let’s ignore the negative numbers and the number zero.
Fun fact: the largest signed 32-bit integer is itself a prime. And the wikipedia page lists it as the 105,097,565th prime.
By the time we hit the 2038 problem, there will have been about 105 million seconds since 1970 where the Unix time was a prime number. And it’s a 10-digit number in base 10, where prime frequency is something about 4% of the numbers.
Does that answer your question about prime frequency today? Eh, I’m sure someone else can figure that out. If not, I’ll probably have to wait until I’m in front of a computer.
