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I stumbled over the following weird behavior when comparing floating-point numbers close to 1.0:

qq = 1.0 + 60.0 $MachineEpsilon;

and then

qq > 1.0

yields False. However,

qq - 1

yields 

1.33227*10^-14

which is far above $MachineEpsilon = 2.22045*10^-16 (the smallest number eps satisfying (1.0 + eps > 1.0)==True when using 8-byte floating point numbers). Changing the first command to

qq = 1.0 + 65.0 $MachineEpsilon;

Leads to the expected behavior of (qq > 1.0) == True.

I'm running Mathematica 10.4.0.0 under Windows on an Intel i5.

Ron

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  • $\begingroup$ SetPrecision[qq, MachinePrecision] > 1 $\endgroup$
    – Feyre
    Commented Dec 6, 2016 at 11:54
  • $\begingroup$ @Feyre Still False. $\endgroup$
    – corey979
    Commented Dec 6, 2016 at 12:02
  • $\begingroup$ @corey979 Sorry, should be 16 or higher, MachinePrecision only works with a qq rationalized as qq = 1 + 60 $MachineEpsilon, something I did without thought, first idea with precision issues is always using rational numbers. $\endgroup$
    – Feyre
    Commented Dec 6, 2016 at 12:06
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    $\begingroup$ This comes about because Equal is fuzzy and mutually exclusive trichotomy is enforced. $\endgroup$
    – Daniel Lichtblau
    Commented Dec 6, 2016 at 14:47
  • 1
    $\begingroup$ I think this "feature" deserves at least a comment in the documentation of $MachineEpsilon. There's a good section in the Help text to Equal[] under "Possible Issues". $\endgroup$
    – RonH
    Commented Dec 6, 2016 at 15:50

1 Answer 1

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$\begingroup$

As said in a comment:

This comes about because Equal is fuzzy and mutually exclusive trichotomy is enforced. – Daniel Lichtblau Dec 6 '16 at 14:47

Perhaps this question also deserves some suggestions for workarounds. The tolerance is relative, so comparing with 0 effectively compares with no tolerance. 

qq - 1.0 > 0
(*  True  *)

The amount of tolerance is controlled by Internal`$EqualTolerance, which can be set to zero. (For a related Q&A, see Machine Epsilon).

Block[{Internal`$EqualTolerance = 0}, qq > 1.0]
(*  True  *)
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  • $\begingroup$ Could you expand a bit on mutually exclusive trichotomy please? $\endgroup$
    – anderstood
    Commented Apr 4, 2018 at 1:21
  • 2
    $\begingroup$ @anderstood The Trichotomy Law states that only one of $a < b$, $a = b$, and $a > b$ can be true. I assume that's what Daniel means. $\endgroup$
    – Michael E2
    Commented Apr 4, 2018 at 1:33
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    $\begingroup$ So could the problem come from the fact that qq==1. returns True? Is qq - 1. == 0 $\neq$ qq == 1. and expected behaviour (I understand that it's a consequence from the relative tolerance). $\endgroup$
    – anderstood
    Commented Apr 4, 2018 at 2:40
  • $\begingroup$ @anderstood Yes, that's why Daniel singles out Equal. $\endgroup$
    – Michael E2
    Commented Apr 4, 2018 at 2:42
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    $\begingroup$ Yes to what I meant by trichotomy and yes to the expected behavior and rationale. $\endgroup$
    – Daniel Lichtblau
    Commented Apr 4, 2018 at 15:54

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