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Werner_E25-Diamond I
25-Diamond I
May 11, 2019
Question

Bug in Mathcads Symbolics

  • May 11, 2019
  • 3 replies
  • 5927 views

Guess this is a bug in Mathcads symbolics or am I missing something?

B.png

    3 replies

    ttokoro
    21-Topaz I
    ttokoro21-Topaz I
    21-Topaz I
    May 11, 2019

    exp-bug.png

    Thanks. This bug must be know.

    t.t.
    W
    Werner_E25-Diamond IAuthor
    25-Diamond I
    May 12, 2019

    The workaround you show does not need the UnitsOf(e) part. It works simply because of the inline evaluation, which redefines e as a simple float number. It looks like e is defined in the symbolics as a function e(x) and this fact should not show through, thats the essence of bug, I guess.

    This would explain to some extent the error message I get - Mathcad seems to think that I am trying to evaluate e(x)(x) which equals e(x,x) but "function" e only accepts just 1 argument, not 2.

    Not sure about that explanation, though.

    B.png

    L
    LucMeekes23-Emerald IV
    23-Emerald IV
    May 12, 2019

    For function g and h the answer for the derivative is now correct. For f it still is NOT.

    See my post below.

     

    Luc

    L
    LucMeekes23-Emerald IV
    23-Emerald IV
    May 12, 2019

    The bug (yes it's a severe bug!), already occurs in the second line/expression.

    The derivative of x*e to x is NOT e(x), but simply e.

    Mathcad11/Maple translates that to exp(1), which is correct to some extent. It's bad in the sense that you cannot have a symbolic variable called e, that means something else than exp(1), but I can live with it. According to ISO 80000, variables should be written in italics, well known mathematics constants and functions should be written in roman (upright) style. Then a variable called e could be anything , but e=exp(1).

    20190512_Bug.png

     

    Is there any difference when defining f(x):=e*x

    And I wonder how Prime fares...it is capable of telling the difference between a variable e and the constant e=exp(1).

     

    Success!
    Luc

     

     

    W
    Werner_E25-Diamond IAuthor
    25-Diamond I
    May 12, 2019

    Is there any difference when defining f(x):=e*x

    And I wonder how Prime fares...it is capable of telling the difference between a variable e and the constant e=exp(1).

     

    No, the order of the multiplication does not matter.

    As I don't care about Prime I did not bother starting up the moloch, but as it seems to be a symbolic bug and Prime uses the same old muPad I guess that there would be not much difference.

     

    Here is another strange effect I noticed while playing around:

    B.png

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    LucMeekes23-Emerald IV
    23-Emerald IV
    May 12, 2019

    The (symbolic) derivative of f(x) to x is exp(2) according to Mathad11/Maple.

    That won't come as a surprise for you I guess.

     

    Luc

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    -MFra-21-Topaz II
    21-Topaz II
    May 13, 2019

    Hello everyone,
    However, if I represent e with exp (1), the result is acceptable:

    xe.jpg

    W
    Werner_E25-Diamond IAuthor
    25-Diamond I
    May 13, 2019

    Yes, thats just another way to force the symbolics into float mode. As shown above this gives a a correct, but only a numeric result - not what we would expect from symbolic math.

    Above I used "e^1.0" and you used the equivalent "exp(1.0)"

    Its bad to know that we can't/shouldn't use the exact e in some symbolic calculations but rather have to resort to a numeric approximation.

    -
    -MFra-21-Topaz II
    21-Topaz II
    May 13, 2019

    but if I use "substitute 2.7182182...=e" , which makes the whole thing a bit redundant, I get the correct symbolic result: 

    xe.jpg

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