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.
