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I undarstand that 1+$MachineEpsilon is actually not equal 1. However, it persists to look like it was equal 1.

In[1]:= 1 + $MachineEpsilon

Out[1]= 1.

This is inappropriate in some cases, as in the following example:

Manipulate[
 Row@{TraditionalForm@HoldForm@Defer@ (1/(x - 1)), "\[Equal]", 
  TraditionalForm@(1/(x - 1))}, {x, 1 + $MachineEpsilon, 2, 
 Appearance -> "Labeled"}]

The above input gives this:

Which very much looks like as if we had some division by zero.

How to fix this? How to display this 1+$MachineEpsilon more accurately?

So far, I've tried this:

In[3]:= N[1 + $MachineEpsilon, $MachinePrecision]

Out[3]= 1.

I fail to understand the above output - 1+$MachineEpsilon is supposed to be a machine number, and therefore it should be accurately representable with $MachinePrecision digits of precision, shouldn't it?

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  • 2
    $\begingroup$ Perhaps SetOptions[$FrontEndSession, PrintPrecision -> 17]? $\endgroup$
    – Michael E2
    Commented Jan 12, 2015 at 0:31
  • $\begingroup$ @MichaelE2 Sadly, no. See: i61.tinypic.com/f5dgrk.jpg The formula result is ridiculously accurate, but 1. persists. $\endgroup$
    – gaazkam
    Commented Jan 12, 2015 at 0:36
  • $\begingroup$ @MichaelE2 Oddly enough, outside Manipulate the precision is correct: i57.tinypic.com/krns3.png $\endgroup$
    – gaazkam
    Commented Jan 12, 2015 at 0:46

2 Answers 2

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I think that the input field that is used to display the current value of x has its own formatting rules and altering them seems difficult. It does not follow PrintPrecision (as in SetOptions[$FrontEndSession, PrintPrecision -> 17], which works for output in an output cell).

So a workaround is to specify the Precision of the displayed x, such that the normal formatting rule shows enough digits. SetPrecision[x, 17] would be sufficient.

Manipulate[
 Row@{TraditionalForm@HoldForm@Defer@(1/(x - 1)), "\[Equal]", 
   TraditionalForm@(1/(x - 1))},
 {x, 1 + $MachineEpsilon, 2, 
   Manipulator[
     Dynamic[SetPrecision[x, 17], (x = #) &], {1 + $MachineEpsilon, 2},
      Appearance -> "Labeled"] &}]

See this answer for some further insight into how the Manipulator is formatted.

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  • $\begingroup$ Many thanks, it works! BTW: SetPrecision[1 + $MachineEpsilon, MachinePrecision + 1] gives 1.0000000000000002, as it should; but SetPrecision[1 + $MachineEpsilon, MachinePrecision] gives 1. Why? 1+$MachineEpsilon is a machine number, so why can't it be displayed with machine precision? And why does N[1 + $MachineEpsilon, 100] still give 1.? $\endgroup$
    – gaazkam
    Commented Jan 12, 2015 at 1:54
  • 1
    $\begingroup$ @gaazkam The precision of 1 + $MachineEpsilon is MachinePrecision, so SetPrecision[1 + $MachineEpsilon, MachinePrecision] does nothing. SetPrecision[x, $MachinePrecision] almost works, but not quite, because 1 + $MachineEpsilon is a little bigger than 1. You could get by with $MachinePrecision + 0.1. It's a bit complicated for a comment, but machine numbers are represented with a finite number of bits and the notion of Precision is relative to the size of x. There are discrete jumps in precision when the exponent changes. N[x, 100] won't increase the precision of x. $\endgroup$
    – Michael E2
    Commented Jan 12, 2015 at 2:46
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Without modifying the precision, you can also just modify the display of the number:

1 + $MachineEpsilon // FullForm
(* 1.0000000000000002` *)

1 + $MachineEpsilon // InputForm
(* 1.0000000000000002 *)

So something like this could work for you:

Manipulate[
  Grid[{{TraditionalForm@HoldForm@Defer@x, "\[Equal]", InputForm@x},
        {TraditionalForm@HoldForm@Defer@(1/(x - 1)), "\[Equal]", TraditionalForm@(1/(x - 1))}}],
  {x, 1 + $MachineEpsilon, 2}]
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