Note that there is a difference between the example in the original email and the answers: The original email was about is_prime(something) not something.is_prime(). I do not know the mechanisms behind is_prime(something) but would it be possible that in this case the behavior is slightly different, say that the input is coerced to ZZ if possible? It would be more user friendly I guess.
Bruno On 5 février 2016 19:59:53 HNEC, William Stein <wst...@gmail.com> wrote: >On Friday, February 5, 2016, David Roe <roed.m...@gmail.com> wrote: > >> >> >> On Fri, Feb 5, 2016 at 1:20 PM, Vincent Delecroix < >> 20100.delecr...@gmail.com >> <javascript:_e(%7B%7D,'cvml','20100.delecr...@gmail.com');>> wrote: >> >>> Hello, >>> >>> Indeed, the definition given in the documentation of "is_prime" does >not >>> coincide with what the method is doing. >>> >>> The mathematical definition of prime *depends* on the ring. An >element of >>> a ring is prime if the ideal it generates is prime. And the ideal >(3) is >>> prime in ZZ but not in QQ. A less stupid example >>> >>> sage: K = ZZ[I] >>> sage: K(3).is_prime() >>> True >>> sage: K(5).is_prime() >>> False >>> >>> The Sage behavior of the function is_prime is perfectly coherent >with >>> respect to this definition. >>> >>> I would rather make a patch to modify the documentation and warns >the >>> user about the difference between prime element in a ring and prime >number. >> >> >> +1 >> David >> > >Maybe is_prime for field elements should just raise an exception? > > > >> >>> >>> Vincent >>> >>> >>> On 05/02/16 14:50, David Wong wrote: >>> >>>> prime_number = bignumber / 2 >>>> is_prime(prime_number) # -> False >>>> >>>> prime_number = bignumber // 2 >>>> is_prime(prime_number) # -> True >>>> >>>> prime_number = ZZ(bignumber / 2) >>>> is_prime(prime_number) # -> True >>>> >>>> >>>> I've spent a couple of days arguing with people about a number >(not) >>>> being >>>> a prime. Turns out it fails silently if you're in the wrong field. >>>> >>>> I guess a fix would be to ZZ() the number first in the function >is_prime. >>>> If you guys agree I can submit a patch or something (never touched >at >>>> Sage's codebase and that would be a good opportunity) >>>> >>>> David >>>> >>>> >>> -- >>> You received this message because you are subscribed to the Google >Groups >>> "sage-devel" group. >>> To unsubscribe from this group and stop receiving emails from it, >send an >>> email to sage-devel+unsubscr...@googlegroups.com >>> ><javascript:_e(%7B%7D,'cvml','sage-devel%2bunsubscr...@googlegroups.com');> >>> . >>> To post to this group, send email to sage-devel@googlegroups.com >>> <javascript:_e(%7B%7D,'cvml','sage-devel@googlegroups.com');>. >>> Visit this group at https://groups.google.com/group/sage-devel. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- >> You received this message because you are subscribed to the Google >Groups >> "sage-devel" group. >> To unsubscribe from this group and stop receiving emails from it, >send an >> email to sage-devel+unsubscr...@googlegroups.com >> ><javascript:_e(%7B%7D,'cvml','sage-devel%2bunsubscr...@googlegroups.com');> >> . >> To post to this group, send email to sage-devel@googlegroups.com >> <javascript:_e(%7B%7D,'cvml','sage-devel@googlegroups.com');>. >> Visit this group at https://groups.google.com/group/sage-devel. >> For more options, visit https://groups.google.com/d/optout. >> > > >-- >Sent from my massive iPhone 6 plus. > >-- >You received this message because you are subscribed to the Google >Groups "sage-devel" group. >To unsubscribe from this group and stop receiving emails from it, send >an email to sage-devel+unsubscr...@googlegroups.com. >To post to this group, send email to sage-devel@googlegroups.com. >Visit this group at https://groups.google.com/group/sage-devel. >For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
