Is this prime? - Hackdot

Re:Is this prime? by thomasahle on Tuesday March 08, 2016 @05:09PM (#11249181)

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.

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Re:Is this prime? by sanderjd on Tuesday March 08, 2016 @11:29PM (#11250938)

I 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!

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Re:Is this prime? by transitorykris on Tuesday March 08, 2016 @10:02PM (#11250603)

Or the Grothendieck prime, 57

https://en.wikipedia.org/wiki/57_(number)

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Re:Is this prime? by cplease on Tuesday March 08, 2016 @11:46PM (#11250997)

Holy shit Grothendieck died in 2014, I NEVER MOURNED HIM. How did this happen?

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Re:Is this prime? by AReallyGoodName on Tuesday March 08, 2016 @06:31PM (#11249663)

The 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.

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Re:Is this prime? by shasta on Tuesday March 08, 2016 @06:05PM (#11249503)

or by 11.

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