Weird thing I apparently have in common with my mother:

[conuly]

I don't remember how it came up, but a few days ago she commented that 51 isn't a prime number, even though it sure feels like it ought to be one.

I feel the exact same way!

And it's really silly, because 51 = 3 x 17 (I memorized it because I got tired of confusing it for a prime number) and it should be obvious with any two digit number whether or not those digits add up to a multiple of 3. (And in a base 10 system, if they do, that means the number is divisible by 3 and, of course, not prime.)

Nevertheless, even after memorizing the factors, I can never quite shake the feeling that 51 ought to be a prime number. And apparently, neither can Mommy.

Comments

[topaz_eyes]

Apparently a product of 2 prime numbers is called a "semiprime." Today I learned!

[crystalpyramid]

Hmm, it's the odd start right? Like it's clearly 21+30, or 81-30, so it's got to be divisible by 3.

21
51
81
111 -- does this feel prime too?
141
171
201

Although with numbers in the hundreds we just don't expect to know in the same way.

[jack]

Yeah, it definitely feels prime to me too, I'm not quite sure why. I think numbers where each digit is prime look more prime even though it doesn't work like that.

[larryhammer]

Math nerd here (I even have a degree to prove it). I stumble on this as well.

[melannen]

31, 41, 61, and 71 are all prime so it just stands to reason!

[conuly]

111 certainly doesn't feel even a little prime, because of course it's a triple digit, and anytime you have the same digit repeated three times it MUST be divisible by three, ergo, not prime.

And of course 201 is just 21 with a 0 in the middle.

[conuly]

11 is prime too, but I'm certain that by definition this does not stand to reason :)

[melannen]

91, annoyingly, also is not. :(

[hilarita]

It doesn't feel much like a prime to me, either :-)

And then there's Groethendieck's Prime - 57. Which is *obviously* 3 less than 60, which is obviously a multiple of 3, but if very famous pure mathematicians don't have a good feeling for what's a prime, I think that's fine for the rest of us.

[reynardo]

It's one of the "sneaky not-primes" that get slipped into tests as "pick the prime number" - 51, 57, 87 and 91, because even if you've learned your times tables up to 12 x 12, these slip just outside them.

[adafrog]

Interesting.