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There is a nice rule I heard once: "Below 100 (119 really), any number that looks like a prime is a prime. Except 49 and 91". The remaining composite numbers a
by thomasahle 11y ago
There is a nice rule I heard once: "Below 100 (119 really), any number that looks like a prime is a prime. Except 49 and 91".
The remaining composite numbers are all trivially divisible by 2 or 5, or their digit sum is divisible by 3.
- shasta 11y agoor by 11.
- AReallyGoodName 11y agoThe not 49 and 91 rule combined with the rule that "a prime above 3 is always exactly 1 off being a multiple of 6" will get you flying through this test.
- transitorykris 11y agoOr the Grothendieck prime, 57 https://en.wikipedia.org/wiki/57_(number) https://en.wikipedia.org/wiki/57_(number)
- cplease 11y agoHoly shit Grothendieck died in 2014, I NEVER MOURNED HIM. How did this happen?
- sanderjd 11y agoI don't think 49 "looks like a prime", because it's clearly 7^2. But I hadn't realized that the other perfect squares are obvious non-primes for other reasons, ie. because of 2s, 3s, and 5s. Neat!