.is_real function
Why does "sqrt(5).is_real()" come up false when it is obviously not? This seems to happen with any square root.
Why does "sqrt(5).is_real()" come up false when it is obviously not? This seems to happen with any square root.
Hi,
Never believe the symbolic ring as it can answer False when the result is True (hopefully a True is real True). To get the answer to your question, you would better do:
sage: QQbar(5).sqrt() in AA
True
Note: QQbar is the set of algebraic number and AA is the set of real algebraic numbers.
Vincent
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Asked: 2014-02-13 23:29:11 +0100
Seen: 625 times
Last updated: Jun 29 '14
There are many problems with this function. - sqrt(5).real_part() -> sqrt(5) - sqrt(5).imag_part() ->0 - sqrt(25).is_real() -> AttributeError: 'sage.rings.integer.Integer' object has no attribute 'is_real' - SR(sqrt(25)).is_real() -> True - SR(sqrt(5)).is_real() -> False - CC(sqrt(5)).is_real -> True
I think this is because `sqrt` gives a symbolic thing, which Sage would have to exert possibly significant computational effort to decide whether it is real (think of showing whether an arbitrary expression is zero) and so answers `False` since it can't prove `True`.
"sqrt gives a symbolic thing, ..." perhaps , but Sage knows that sqrt(5).imag_part() is 0 then it isn't so difficult to answer True at this question. But I have only notions of programming.