# [Maxima] realonly in algsys.lisp

Alexey Beshenov al at beshenov.ru
Sun Apr 26 05:05:35 CDT 2009

```The option variable realonly is quite confusing since it does not find
real solutions, but only solutions which are free of %i (purely using
freeof).

For example, when realonly is false,

algsys ([x^4 + 1], [x]);

returns

[[x = (-1)^(1/4)],  [x = -(-1)^(1/4)*%i],
[x = -(-1)^(1/4)], [x = (-1)^(1/4)*%i]]

But when realonly is true, it returns

[[x = (-1)^(1/4)], [x = -(-1)^(1/4)]]

while it is natural to expect [].

Maybe it is better to modify the behavior and filter roots by
checking something like

is_real(x) := is(trigsimp(imagpart(x)) = 0);

and not just

freeof(%i, x)

With the freeof approach we may also omit real roots of irreducible
polynomials:

sol : map ('rhs, solve (3*x^3  - 3*x + 1));
[(sqrt(3)*%i/2-1/2)/(3*(3^-(3/2)*%i/2-1/6)^(1/3))
+(3^-(3/2)*%i/2-1/6)^(1/3)*(-sqrt(3)*%i/2-1/2),
(3^-(3/2)*%i/2-1/6)^(1/3)*(sqrt(3)*%i/2-1/2)
+(-sqrt(3)*%i/2-1/2)/(3*(3^-(3/2)*%i/2-1/6)^(1/3)),
(3^-(3/2)*%i/2-1/6)^(1/3)+1/(3*(3^-(3/2)*%i/2-1/6)^(1/3))]

map (lambda([x], freeof(%i, x)), sol);
[false,false,false]

map ('is_real, sol);
[true,true,true]

So when realonly and algexact are set to true,

algsys ([3*x^3  - 3*x + 1], [x])

just returns [].

What do you think?

--
Yours sincerely,