91–100 of 139 posts
Earlier quoted context omitted.
Also, most composites are divisible by 2, 3 or 5, all of which are very easy to check (for 3 just sum up the digits and if the result is divisible by 3, the original number is divisible by 3). 49, 77, and 91 are the only composites under 100 that is not divisible by one of those 3 numbers, making memorizing 25 numbers not even necessary. 49 and 77 are obviously composite to me making 91 the only number I bothered mem…
53?
When I was a fifth grader in China, we were required to memorize all prime numbers below 100. Curious if that is common in other countries?
When I was a fifth grader we were required to memorize all prime numbers below 20.
I've spent a good week of straight hours on primes by now, the most fun thing I've come up with was this boring sieve. https://github.com/alexlieberman/Prime-Numbers/blob/master/S...
78. Who did better?
Here is a "theorem" I learned: every number up to 100 which looks prime, is prime, except 91. Does anyone recall its name?
I agree. And I would add 51 to that.
51 is #1, then 57, then 1
function c(){is_prime(document.getElementById('n').textContent)==="prime"?yes.click():no.click();window.setTimeout(c,1)};c()
This made me sad :(
If I click start, then paste some quick and dirty browser automation in the console: function c(){is_prime(document.getElementById('n').textContent)==="prime"?yes.click():no.click();window.setTimeout(c,1)};c() My old laptop can guess right 16k times and get up to 4172973243025599, but then the http post to do the stats (record.php) bombs with a MySql error "Out of range value for column 'end' at row 1" :)