#4942: find_root() is broken when interval borders cannot be evaluated
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Reporter: mabshoff | Owner: mhansen
Type: defect | Status: assigned
Priority: blocker | Milestone: sage-3.3
Component: numerical | Resolution:
Keywords: |
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Comment (by cwitty):
There are at least a couple of issues here. First, brentq is a variant of
a bisection-based solver; if you use any bisection-based solver to find a
zero of 1/(x-1) between 0 and 2, it will narrow down and return something
very close to 1. So if we don't like that, we should use a different
solver (or at least try to check the output; for instance, a simple check
that f(x) is "small" would detect this particular problem).
Second, find_root tries to verify that the function evaluates to different
signs at the endpoints of the interval (as required by brentq); but it
doesn't check the function evaluation results for NaN. In the original
test case, fast_float(f)(0) gives NaN.
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/4942#comment:5>
Sage <http://sagemath.org/>
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