[Octave-bug-tracker] [bug #43041] isprime(5i) returns true

[Jordi Gutiérrez Hermoso]

Follow-up Comment #5, bug #43041 (project octave):

It is convenient for many purposes to define a prime number as a non-unit
number that divides a factor whenever it divides a product (a unit is a number
that divides 1). This is the definition that generalises to any integral
domain. Following this definition, both Maxima's and GAP's isprime() and
IsPrimeInt() respectively consider negative prime numbers to be prime.
Interestingly, Pari/GP, which is actually specialised for number theory,
excludes negative prime numbers from its isprime() function.

Since I consider "Matlab compatibility" to allow not erroring out when Matlab
errors out, and since Matlab errors out on negative integers, I think we can
pick whichever definition works out. I prefer to consider negative primes as
primes, but if you feel strongly otherwise, feel free to make the change to
exclude them or to further document my implementation.

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