Re: [sage-support] something seems wrong here
2022-01-02 00:00:56 UTC, Michael Beeson on sage-devel: > sage: d = 6 > sage: p = d/2 > sage: p > 3 > sage: is_prime(p) > False # Huh?!! > sage: is_prime(3) > True > sage: p==3 > True > > This happens in version 8.7 and also in the current version (installed yesterday) William Stein answered: > The parent of p is QQ (the rational field). > p is not a prime number of the **rational field**. > You should coerce p to ZZ first. To change `p` to an integer before testing whether it is prime: ``` sage: d = 6 sage: parent(d) Integer Ring sage: p = d/2 sage: parent(p) Rational Field sage: p = ZZ(p) sage: is_prime(p) True ``` Or, to take quotients in the integers from the start, use ``` sage: d = 6 sage: p = d // 2 sage: parent(p) Integer Ring sage: is_prime(p) True ``` The behaviour of `is_prime` for field elements is the object of - Sage Trac ticket #17919 Disallow is_prime() for FieldElement https://trac.sagemath.org/ticket/17919 - Sage Trac ticket #32321 Make is_prime() return true for rational numbers that are prime as integers https://trac.sagemath.org/ticket/32321 See also: - Sage Trac ticket #32340 Document behavior of .is_prime() for number fields https://trac.sagemath.org/ticket/32340 - Sage Trac tickets whose summary contains is_prime https://trac.sagemath.org/query?order=id&desc=1&summary=~is_prime -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-support/74b4969e-5fd1-4612-9b2b-6a89d95cd1c2n%40googlegroups.com.
Re: [sage-support] something seems wrong here
More here: https://doc.sagemath.org/html/en/reference/structure/sage/structure/element.html#sage.structure.element.RingElement.is_prime On Sat, Jan 1, 2022 at 4:27 PM William Stein wrote: > > On Sat, Jan 1, 2022 at 4:01 PM Michael Beeson wrote: > > > > sage: d = 6 > > > > sage: p = d/2 > > > > sage: p > > > > 3 > > > > sage: is_prime(p) > > > > False # Huh?!! > > The parent of p is QQ (the rational field). > p is not a prime number of the **rational field**. > You should coerce p to ZZ first. > > In retrospect, we probably should have just made is_prime throw an > exception if the parent ring is a field, since the ring theory > definition of prime number isn't very useful in a field. > > > > > sage: is_prime(3) > > > > True > > > > sage: p==3 > > > > True > > > > This happens in version 8.7 and also in the current version (installed > > yesterday) > > > > > > -- > > You received this message because you are subscribed to the Google Groups > > "sage-support" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to sage-support+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/sage-support/9cb13872-298c-4463-b27d-b77ea288bfedn%40googlegroups.com. > > > > -- > William (http://wstein.org) -- William (http://wstein.org) -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-support/CACLE5GDVrKCa8-SCn-2p22oXDyQgtnS6s26upXCA-z%2Bx9EZAuw%40mail.gmail.com.
Re: [sage-support] something seems wrong here
On Sat, Jan 1, 2022 at 4:01 PM Michael Beeson wrote: > > sage: d = 6 > > sage: p = d/2 > > sage: p > > 3 > > sage: is_prime(p) > > False # Huh?!! The parent of p is QQ (the rational field). p is not a prime number of the **rational field**. You should coerce p to ZZ first. In retrospect, we probably should have just made is_prime throw an exception if the parent ring is a field, since the ring theory definition of prime number isn't very useful in a field. > > sage: is_prime(3) > > True > > sage: p==3 > > True > > This happens in version 8.7 and also in the current version (installed > yesterday) > > > -- > You received this message because you are subscribed to the Google Groups > "sage-support" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sage-support+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sage-support/9cb13872-298c-4463-b27d-b77ea288bfedn%40googlegroups.com. -- William (http://wstein.org) -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-support/CACLE5GDnGOFcb2OEO3OCczOuj-Yvzxa-k%3Dvx0xfoYog52Mogow%40mail.gmail.com.
