GNU bug report logs - #58929
29.0.50; Calc: finding roots utpn doesn't work as advertised

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Message #8 received at 58929 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: Matt Armstrong <matt <at> rfc20.org>
Cc: 58929 <at> debbugs.gnu.org
Subject: bug#58929: 29.0.50; Calc: finding roots utpn doesn't work as
 advertised
Date: Tue, 1 Nov 2022 17:51:11 +0100

Unfortunately straight Newton won't converge for this function (erf, essentially) if you stray too far from the origin (ie, the mean) so I suggest you always use that as a starting guess no matter what point you are trying to find the inverse for.

Another way is to use an interval as starting guess instead of a single point; that seems to make Calc switch tactics and persevere.

One thing that can happen is running out of stack, because many of the implementations like math-newton-root are recursive. This should be easy to remedy as they all seem to be tail calls.

Obviously the claim to guarantee a solution for "any initial guess" is fanciful.

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