On Wed, Feb 19, 2020 at 2:25 PM Varun Rajkumar <pi.fermet@gmail.com> wrote:

Bill, my eulercircle class is doing combinatorics. But your email was not asking that question.

Varun, you may be right. My question was " What is the probability that N is prime if it's coprime to D! ?" What subject is this if not Combinatorics? Number Theory? Combinatoric Number Theory?? —Bill On Thu, Feb 13, 2020 at 9:30 PM Rudy Rucker <rudy@rudyrucker.com> wrote:

From the peanut gallery.

When I can't sleep I factor all the numbers less than a hundred, usually working down from 99. And going off on tangents. And if I went to sleep in the 70s the day before I restart there. If I feel extra ambitious I jump up into numbers less than 200, but blessed sleep soon comes. In the same vein, I had a bout of computing larger and larger Fibonacci series numbers this fall.

Factoring 5 and 6 digit numbers in my head? Not yet... On 2/13/2020 7:43 PM, Bill Gosper wrote:

Here's a question I often ask myself but am too lazy to attack. (And I'm working on a 3D print.) I like to factor 5 or 6 digit numbers in my head. There's a bag of tricks for finding small factors, but eventually you wind up slogging through division by consecutive primes. When I've reached, say, ¾ √N, I often get bored|tired and then cheat by trying PrimeQ, and then when it says True, I'm glad I cheated. It seems to me that this happens improbably often. What is the probability that N is prime if it's coprime to D! ? —Bill

-- Rudy Rucker | rudy@rudyrucker.com |

On Thu, 13 Feb 2020 at 22:48, Bill Gosper <billgosper@gmail.com> wrote:

...

http://www.purplestreak.com/g4g13/mentalfactoring.pdf

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