[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,
Alexey Beshenov <al at cadadr.org>
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