maxima-bugs

[Maxima-bugs] [maxima:bugs] #4636 signum(ind) is an error

[Barton W. <wil...@us...>]

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**[bugs:#4636] signum(ind) is an error**

**Status:** open
**Group:** None
**Labels:** extended real signum limit 
**Created:** Thu Nov 27, 2025 12:41 PM UTC by Barton Willis
**Last Updated:** Thu Nov 27, 2025 12:41 PM UTC
**Owner:** nobody


Either a  `signum` nounform or `ind` is a better result than an error:
~~~
(%i4)	signum(ind);
sign: sign of ind is undefined.
~~~
Should the general simplifier, or the one‑argument limit function, handle` F(extended-real)`? We’ve discussed this—do we have a consensus?



---

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[Maxima-bugs] [maxima:bugs] #4636 signum(ind) is an error

[Barton W. <wil...@us...>]

I simple fix is to errcatch sign.  I addition to a few minor edits, I used the fact that `complex-number-p` returns multiple values to make the complex number case a bit more efficient.

Let me know what you all think of this revision.

~~~
;; This is missing signum(x^n) = signsum(x)^n, where n is an integer--possibly should be restricted to a positive integer.
(def-simplifier signum (x)
  (cond
    ;; Case: zero--this makes signum(0.0) -> 0.0 and signum(0.0b0) -> 0.0b0
    ((zerop1 x) x)
    (t
     (multiple-value-bind (is-complex-number re im)
         (complex-number-p x #'mnump)
       (cond
         ;; Case: complex number
         (is-complex-number
          ($expand (div x (ftake 'mexpt (add (mul re re) (mul im im)) 1//2)) 1 0))
         ;; Idempotent: signum(signum(z)) = signum(z)
         ((and (consp x) (eq '%signum (caar x))) x)       
         (t
          ;; We use the errcatch to avoid signum(ind) throwing an error
          (let ((sgn (car (errcatch ($csign x)))))
            (cond
              ;; signum(negative) -> -1
              ((eq sgn '$neg) -1)
              ;; signum(zero) -> 0
              ((eq sgn '$zero) 0)
              ;; signum(positive) -> 1
              ((eq sgn '$pos) 1)
              ;; Multiplicative: signum(ab) = signum(a) * signum(b)
              ((mtimesp x)
               (fapply 'mtimes (mapcar #'(lambda (s) (ftake '%signum s)) (cdr x))))
              ;; Reflection rule: signum(-x) -> -signum(x)
              ((great (neg x) x)
               (neg (ftake '%signum (neg x))))
              ;; Give up
              (t (give-up))))))))))
~~~


---

**[bugs:#4636] signum(ind) is an error**

**Status:** open
**Group:** None
**Labels:** extended real signum limit 
**Created:** Thu Nov 27, 2025 12:41 PM UTC by Barton Willis
**Last Updated:** Thu Nov 27, 2025 12:41 PM UTC
**Owner:** nobody


Either a  `signum` nounform or `ind` is a better result than an error:
~~~
(%i4)	signum(ind);
sign: sign of ind is undefined.
~~~
Should the general simplifier, or the one‑argument limit function, handle` F(extended-real)`? We’ve discussed this—do we have a consensus?



---

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[Maxima-bugs] [maxima:bugs] Re: #4636 signum(ind) is an error

[Stavros M. <mac...@us...>]

Not sure when this code got in here:
   ($expand (div x (ftake 'mexpt (add (mul re re) (mul im im)) 1//2)) 1 0))
but that seems like a silly way to handle the real case (and makes the rest of the code unnecessary for real numerical inputs). Presumably this should not be used when im=0.


---

**[bugs:#4636] signum(ind) is an error**

**Status:** open
**Group:** None
**Labels:** extended real signum limit 
**Created:** Thu Nov 27, 2025 12:41 PM UTC by Barton Willis
**Last Updated:** Fri Nov 28, 2025 01:01 PM UTC
**Owner:** nobody


Either a  `signum` nounform or `ind` is a better result than an error:
~~~
(%i4)	signum(ind);
sign: sign of ind is undefined.
~~~
Should the general simplifier, or the one‑argument limit function, handle` F(extended-real)`? We’ve discussed this—do we have a consensus?



---

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[Maxima-bugs] [maxima:bugs] #4636 signum(ind) is an error

[Barton W. <wil...@us...>]

I inserted the code  `($expand (div x ... ) 1 0)`. I didn't like it too much, but it I did it to avoid a check if the imaginary part was zero and make sure that the complex number case returns a number in rectangular form.  

I'll revise this bit of code.


---

**[bugs:#4636] signum(ind) is an error**

**Status:** open
**Group:** None
**Labels:** extended real signum limit 
**Created:** Thu Nov 27, 2025 12:41 PM UTC by Barton Willis
**Last Updated:** Fri Nov 28, 2025 01:14 PM UTC
**Owner:** nobody


Either a  `signum` nounform or `ind` is a better result than an error:
~~~
(%i4)	signum(ind);
sign: sign of ind is undefined.
~~~
Should the general simplifier, or the one‑argument limit function, handle` F(extended-real)`? We’ve discussed this—do we have a consensus?



---

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[Maxima-bugs] [maxima:bugs] #4636 signum(ind) is an error

[Barton W. <wil...@us...>]

One more reason: I wanted to make `signum(2.0b0) = 1.0b0, signum(2/3) = 1, ...`.  the silly code was an easy way to achieve that


---

**[bugs:#4636] signum(ind) is an error**

**Status:** open
**Group:** None
**Labels:** extended real signum limit 
**Created:** Thu Nov 27, 2025 12:41 PM UTC by Barton Willis
**Last Updated:** Fri Nov 28, 2025 01:21 PM UTC
**Owner:** nobody


Either a  `signum` nounform or `ind` is a better result than an error:
~~~
(%i4)	signum(ind);
sign: sign of ind is undefined.
~~~
Should the general simplifier, or the one‑argument limit function, handle` F(extended-real)`? We’ve discussed this—do we have a consensus?



---

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