[Maxima-bugs] [ maxima-Bugs-1667251 ] cf not correct for composed expressions

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Bugs item #1667251, was opened at 2007-02-23 11:42
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Category: Lisp Core
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Stavros Macrakis (macrakis)
Assigned to: Nobody/Anonymous (nobody)
Summary: cf not correct for composed expressions

Initial Comment:
cf is grossly wrong for composed expressions.  For example:

 cf(sqrt(2)*sqrt(3)), cflength=3 => [2,2,4,1,5,1,1,3]
 cf(sqrt(6)), cflength=3 =>         [2,2,4,2,4,2,4]
                                           ^^
This is because it first calculates the truncated cf of the parts, and multiplies them. There is no way a priori of knowing which terms in the result are correct. There also seem to be some egregious bugs (which aren't worth fixing because of the above general problem):

        expr: sqrt(3)-sqrt(2)$
        float(expr) =>  0.318...
        cf(expr),cflength=5 => [3, 6, 1, 5, 7, ...
        cf(float(expr)) => [0,3,6,1,5...]
        float(cfdisrep(cf(expr))), cflength=5 => 3.146...
        float(cfdisrep(cf(expr))), cflength=100 => 3.146...

I do not see the usefulness of calculating cf(sqrt(2)*sqrt(3)) as cf(sqrt(2))*cf(sqrt(3)), but perhaps someone else does.  I also don't see the usefulness of the cflength parameter as currently defined. 

More examples where current code is wrong (and also produces lots of spurious extra terms):

cf(sqrt(2/3)),cflength:12 =>
  [0,1,4,2,4,2,4,2,4,2,3,1,281,4,2,1,9,1,3,2,2,14]
                      ^^^ first incorrect term @ position 11
          (would be correct if rounded to 11 terms, but not to 12)

(With Andreas Eder's new code):
cf((3/5)^(1/3)),cflength:5 =>
  [0, 1, 5, 3, 2, 2, 2]
            ^^^ first incorrect term @ position 4
cf(100000^(1/3)),cflength:5 == cflength(10*100^(1/3)) =>
  [46,2,2,2,8,1,1 ...]
        ^ first incorrect term @ 3



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