tag:blogger.com,1999:blog-1141932539860553199.post5816556497576661188..comments2024-03-05T10:52:24.691+00:00Comments on Mark Wadsworth: Checking whether a number is prime. Mark Wadsworthhttp://www.blogger.com/profile/07733511175178098449noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-1141932539860553199.post-23429147541729428092021-05-26T19:44:10.383+01:002021-05-26T19:44:10.383+01:00VFTS, mod 30 rule is not an official thing. I just...VFTS, mod 30 rule is not an official thing. I just cobbled it together.<br /><br />D, correct. You could do mod 210, or mod 2,310.Mark Wadsworthhttps://www.blogger.com/profile/07733511175178098449noreply@blogger.comtag:blogger.com,1999:blog-1141932539860553199.post-12029498661584081882021-05-26T18:59:00.740+01:002021-05-26T18:59:00.740+01:00Well, 30 is 2*3*5. That suggests to me that there ...Well, 30 is 2*3*5. That suggests to me that there should be a 210 (2*3*5*7) rule which on the plus side gives even better odds but on the minus side requires you to memorise all the primes up to 209.Derekhttps://www.blogger.com/profile/06296053477905542366noreply@blogger.comtag:blogger.com,1999:blog-1141932539860553199.post-40594450265790426362021-05-26T18:46:16.687+01:002021-05-26T18:46:16.687+01:00mod 30 rule. Hmm, not sure. I'm more than a bi...mod 30 rule. Hmm, not sure. I'm more than a bit rusty on number theory. View from the Solenthttps://www.blogger.com/profile/00930587784230398787noreply@blogger.comtag:blogger.com,1999:blog-1141932539860553199.post-38938324971164386132021-05-26T17:44:45.183+01:002021-05-26T17:44:45.183+01:00VFTS, having thought about it while doing somethin...VFTS, having thought about it while doing something else, where my suggested method gives you a slight edge over "adding digits" is numbers where the remainder is a prime.<br /><br />For example, 1003 = 990 + 13. This tells you it can't divide by 13, so when you are grinding it out, you don't need to check whether it divides by 13 (it divides by 19 as it happens).<br /><br />Counter-example, 1,001 = 990 + 11. If you don't notice that 990 divides by 11, you've missed the point.Mark Wadsworthhttps://www.blogger.com/profile/07733511175178098449noreply@blogger.comtag:blogger.com,1999:blog-1141932539860553199.post-77999346701947049382021-05-26T17:00:17.306+01:002021-05-26T17:00:17.306+01:00VFTS, sure, that method will rule out all numbers ...VFTS, sure, that method will rule out all numbers that divide by 3. Does it exclude the same numbers as the mod 30 rule? Maybe it does and I wasted my time :-)Mark Wadsworthhttps://www.blogger.com/profile/07733511175178098449noreply@blogger.comtag:blogger.com,1999:blog-1141932539860553199.post-58703907063807303562021-05-26T14:54:13.018+01:002021-05-26T14:54:13.018+01:00Mark,
your '3-test' can be extended to avo...Mark,<br />your '3-test' can be extended to avoid any dividing.<br />If you sum the original integers, sum the integers in the result, and so on until you finish with a single digit. If that digit is 3, 6, 9, the original is not primeView from the Solenthttps://www.blogger.com/profile/00930587784230398787noreply@blogger.com