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Here is a "theorem" I learned: every number up to 100 which looks prime, is prime, except 91. Does anyone recall its name?
by ridiculous_fish 5y ago
Here is a "theorem" I learned: every number up to 100 which looks prime, is prime, except 91. Does anyone recall its name?
- jkingsbery 5y agoI think it's called "The Ridiculous Fish Theorem." +1... I lost two games in a row, both times on 91.
- nilstycho 5y agoThis is a great "theorem", thank you! It's easy to determine divisibility by 2, 3, 5, and 11. 7² is also easy because it's a square. 7×13=91 is the only composite number under 100 that isn't caught by these rules.
- deleted 5y ago[deleted]
- deleted 5y ago[deleted]
- bbx 5y agoI agree. And I would add 51 to that.
- TheDong 5y ago51's an easy one because 5+1=6, and 6 is divisible by 3, so it must be divisible by 3. It's easy to try 'divisible by 5' (ends in 5 or 0) and 'divisible by 3' (sum of digits is divisible by 3). 91 isn't found as prime by those two tests, so it needs the extra exceptional rule.
- bbx 5y agoOf course! I actually know of this divisible by 3 rule, and use it to test out all numbers, like 87, 111, or 69, but somehow forget to apply it to 51…
- Tempest1981 5y agoThey have stats on the most common mistakes here: https://isthisprime.com/game/record.php https://isthisprime.com/game/record.php (loads very slowly) 51 is #1, then 57, then 1
- nilstycho 5y agoIt’s very surprising to me that 91 isn’t in the top 20. It’s beaten by 77 and 21?
- tshaddox 5y ago49 looks prime to me. Another commentator says it doesn't count because it's a square number, but square numbers don't really have a particular "look" to me in the same way that 2-digit numbers ending in 5 or with digits summing to a multiple of 3 do.
- aidenn0 5y agoYour maths teacher didn't force you to memorize squares then.
- tshaddox 5y agoI did memorize the sequence and still have it memorized, but I have to enumerate them. They don't have a "look" to them.
- crackercrews 5y agoThere's no "look" for me either. But I immediately know that 49 is not prime because I recognize it as the answer to one of the times table math problems that I learned.
- jcims 5y agoBased on my failure at this game, anything times 19 looks prime to me.
- Dylan16807 5y agoWell you only have 4 of those under 100. The "looks prime" rule assumes you check for multiples of 3, so that removes one. All the rest end in an even digit or 5.
- jcims 5y agoyeah once i started doing the trick for 3's the whole thing got boringly easy
- credit_guy 5y agoDon’t know the name of the “theorem”, but I know the rhyme: 91 is not a prime that one trips me every time.
- tobr 5y agoBut that doesn’t really help you remember 91, because the number is not part of the rhyme.
- hanoz 5y agoReally like that! Presumably this is because most people's sense of what feels prime is: odd numbers which don't appear in the times tables; and only 39, 51, 57, 69, 87, 91 and 93 are red herrings in that regard, all of which bar 91 are divisible by 3, which people also have a good sense of, not least by using the add the digits rule.
- marcan_42 5y agoDammit, that's the one that got me on my first try.
- asddubs 5y agoi got got by 67 on my first try. something about it just gave me non-prime vibes.