#4892: Changing precision of a Complex can convert it to a real
------------------------------+---------------------------------------------
Reporter: cremona | Owner: somebody
Type: defect | Status: new
Priority: major | Milestone: sage-3.4
Component: basic arithmetic | Keywords: real complex precision
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georg.grafendorfer reported this to sage-support:
{{{
sage: a = CC(-5).n(prec=100)
sage: b = ComplexField(100)(-5)
sage: a == b
True
sage: type(a) == type(b)
False
sage: ln(a)
NaN
sage: ln(b)
1.6094379124341003746007593332 + 3.1415926535897932384626433833*I
}}}
The issue is that both a and b are equal to -5 (exactly, to 100 bit
precision) but a is a Real while b is a Complex. This happens because
{{{
Looking at the code for numerical_approx() in sage.misc.functional,
this happens because the attempt to coerce z into RealField(100)
succeeds. To fix this (if it is agreed that it is a bug) that
function would need to test the type of the input and return something
of the same type (real/complex).
}}}
The suggested fix is that the numerical_approx function should always
return a complex number to the appropriate precsion if the input has type
complex, even if the coercion into a real succeeded.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/4892>
Sage <http://sagemath.org/>
Sage - Open Source Mathematical Software: Building the Car Instead of
Reinventing the Wheel
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