#299 2/sqrt(2) doesn\'t simplify

closed
nobody
8
2009-06-23
2003-04-15
No

2/sqrt(2) doesn't simplify. Similarly for 2/2^(2/3).

On the other hand, x/sqrt(x) => sqrt(x).

And of course sqrt(2) simplifies to itself -- it doesn't
become 2/sqrt(2)!!

I believe the original examples should simplify to sqrt(2)
and 2^(1/3). Note that 2^(4/3) => 2*2^(1/3) (the current
behavior) is probably CORRECT, in order to make things
like 10^(10/3) intelligible.

Or is there something I'm missing?

Maxima 5.9.0 gcl 2.5.0 mingw32 Windows 2000 Athlon

Discussion

  • Barton Willis

    Barton Willis - 2003-04-17

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    Try ratsimp with algebraic : true

    (C1) z : 2/sqrt(2);

    (D1) 2/SQRT(2)

    (C2) ratsimp(z);

    (D2) 2/SQRT(2)

    (C3) ratsimp(z),algebraic;

    (D3) SQRT(2)

    (C4) z : 2/2^(2/3);

    (D4) 2/2^(2/3)

    (C5) ratsimp(z);

    (D5) 2/2^(2/3)

    (C6) ratsimp(z),algebraic;

    (D6) 2^(1/3)
    (C7)

     
  • Stavros Macrakis

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    Yes, of course there are ways within Maxima to perform this
    simplification. But it should be the default in the general
    simplifer. The logic already appears to be in the general
    simplifier, but there is a bug in this particular case. If the
    general simplifier's philosophy were to leave such things
    untouched, why does it simplify x/sqrt(x) and the like?

     
  • Stavros Macrakis

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    More examples. Right-hand side is after ratsimp/algebraic. I
    believe the general simplifier should be giving those forms.

    1/(2*2^(2/3)) 2^(1/3)/4
    1/2^(2/3) 2^(1/3)/2
    1/(2*SQRT(2)) SQRT(2)/4
    1/SQRT(2) SQRT(2)/2
    1/(2*2^(1/3)) 2^(2/3)/4
    1/2^(1/3) 2^(2/3)/2

    Things get worse with non-numeric contents. In the
    following, each group of expressions denotes the same thing,
    but none simplifies to the others. I have put *** next to
    those forms which are the results of ratsimp/algebraic. Note
    that in several cases, there is more than one equivalent
    ratsimp'ed form....

    1/(a*b)^(5/2)
    1/(a^2*b^2*SQRT(a*b)) ***
    SQRT(a*b)/(a^3*b^3) ***

    1/(a*b)^(3/2)
    1/(a*b*SQRT(a*b)) ***
    SQRT(a*b)/(a^2*b^2) ***

    1/(a*b)^(7/6)
    1/(a^(2/3)*b^(2/3)*SQRT(a*b)) ***
    SQRT(a*b)/(a^(5/3)*b^(5/3)) ***
    (a*b)^(5/6)/(a^2*b^2) ***

    1/(a*b)^(5/6) ***
    1/(a^(1/3)*b^(1/3)*SQRT(a*b)) ***
    (a*b)^(1/6)/(a*b) ***
    SQRT(a*b)/(a^(4/3)*b^(4/3)) ***

    1/SQRT(a*b) ***
    SQRT(a*b)/(a*b) ***

    a^(1/3)*b^(1/3)/SQRT(a*b) ***
    1/(a*b)^(1/6) ***
    SQRT(a*b)/(a^(2/3)*b^(2/3)) ***
    (a*b)^(5/6)/(a*b) ***

    Now it is true that these expressions are in fact not all
    equivalent as to principal value, but I will leave that exercise
    for later. Many of them are, and they are not being
    canonicalized.

     
  • Robert Dodier

    Robert Dodier - 2006-07-06
    • labels: --> 460522
     
  • Robert Dodier

    Robert Dodier - 2006-08-27
    • labels: 460522 --> Lisp Core - Simplification
    • summary: 2/sqrt(2) doesn't simplify --> 2/sqrt(2) doesn\'t simplify
     
  • Stavros Macrakis

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    I have raised the priority of this bug, because it is very close to the surface (i.e. easy for just about any user to run into). See also 1853191, where algebraic gives strange results...

     
  • Stavros Macrakis

    • priority: 5 --> 8
     
  • Dan Gildea

    Dan Gildea - 2008-01-11

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    (%i6) (1/2)*sqrt(2);
    (%o6) sqrt(2)/2

    (%i7) sqrt(2)*(1/2);
    (%o7) 1/sqrt(2)

     
  • Raymond Toy

    Raymond Toy - 2008-03-28

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    The issue appears to be in timesin. The basic issue is that timesin isn't commutative, as dgildea shows.

    I have a partial fix for this which fixes this particular issue, but it causes many failures in the testsuite, mostly due to a different ordering of the answer. But some tests now cause errors to be signaled, and some are no longer simplified as before or not simplified at all.

    Bummer.

     
  • Dieter Kaiser

    Dieter Kaiser - 2009-06-23
    • status: open --> closed
     
  • Dieter Kaiser

    Dieter Kaiser - 2009-06-23

    With revision 1.80 of simp.lisp the initial problem simplifies as expected:

    (%i6) 2/sqrt(2);
    (%o6) sqrt(2)

    Furthermore we have:

    (%i7) 1/sqrt(2)*2;
    (%o7) sqrt(2)

    (%i8) (1/2)*sqrt(2);
    (%o8) 1/sqrt(2)

    (%i9) sqrt(2)/2;
    (%o9) 1/sqrt(2)

    Closing this bug report as fixed.

    Dieter Kaiser

     

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