[Maxima] Integrator loops endlessly for lambert_w(1/x)

Dieter Kaiser drdieterkaiser at web.de
Thu Dec 25 22:29:41 CST 2008


Today I have recognized that David Billinghurst has added a test to
rtest_lambert_w.mac which fails: 

  integrate(lambert_w(1/x),x)

The integrator loops endlessly.

I had a long search for the problem. The integrator tries to do an
integration-by-parts and generates the following integrand:

%e^(-lambert_w(1/x))/(x*(1+lambert_w(1/x)))

This is the expression we get when we type it in at the Maxima prompt
(The numbers enumerate the different parts of the expression):

   ((MTIMES SIMP) 
1*  ((MEXPT SIMP) $X -1) 
2*  ((MEXPT SIMP) $%E 
     ((MTIMES SIMP) -1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))))
3*  ((MEXPT SIMP) 
     ((MPLUS SIMP) 1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))) -1))

Then I have looked into the integrator. In the integrator the expression
is a bit different ordered. The expression is generated in the routine
partial-integration and transfered to the integrator. This is the
expression the integrator gets and tries to solve:

(%i7) :lisp (setq $form 
   '((MTIMES SIMP)
3*   ((MEXPT SIMP)
      ((MPLUS SIMP) 1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))) -1)
1*   ((MEXPT SIMP) $X -1)
2*   ((MEXPT SIMP) $%E
      ((MTIMES SIMP) -1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))))))

If the routine intform finds a mexpt expression with a complicated
exponent it tries the following code:

(let* (($%e_to_numlog t)
       (nexp (resimplify exp))) ; resimplify the whole integrand
  (cond ((alike1 exp nexp) nil) ; no change, return nil
        (t 
         (when *debug-integrate*
           (format t "~& in INTFORM: NEW EXP is ~A~%" nexp))
         (intform (setq exp nexp)))))))) ; try again with new exp

This code is involved doing the integral which fails. Now what happens
is that we get for every resimplify a new expression. The condition
(alike1 exp nexp) is never true and the integrator loops endlessly.

I have tried it directly at the Maxima prompt. Here are the results:

This is a first resimplify for the expression from above. Look at the
numbers:

(%i7) :lisp (resimplify $form)

   ((MTIMES SIMP)
2*  ((MEXPT SIMP) $%E 
     ((MTIMES SIMP) -1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))))
3*  ((MEXPT SIMP) 
     ((MPLUS SIMP) 1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))) -1)
1*  ((MEXPT SIMP) $X -1))

We resimplify two times:

(%i7) :lisp (resimplify (resimplify $form))

   ((MTIMES SIMP)  
1*  ((MEXPT SIMP) $X -1)
2*  ((MEXPT SIMP) $%E 
     ((MTIMES SIMP) -1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))))
3*  ((MEXPT SIMP) 
     ((MPLUS SIMP) 1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))) -1))

And again:

(%i7) :lisp (resimplify (resimplify (resimplify $form)))

   ((MTIMES SIMP)
3*  ((MEXPT SIMP) 
     ((MPLUS SIMP) 1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1))) -1)
1*  ((MEXPT SIMP) $X -1)
2*  ((MEXPT SIMP) $%E
      ((MTIMES SIMP) -1 ((%LAMBERT_W SIMP) ((MEXPT SIMP) $X -1)))))

It seems to me that we have got by accident an expression which does not
simplify as expected and change the form every time we do a further
resimplify.

At the moment I have no idea how to continue.

Dieter Kaiser



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