From: "Mattias Engdegård" <mattiase@acm.org>
To: Matt Armstrong <matt@rfc20.org>
Cc: 58929@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 [thread overview]
Message-ID: <8773828B-3A5F-4B15-8786-91F6FCB6D2E2@acm.org> (raw)
In-Reply-To: <87fsf3uanl.fsf@rfc20.org>
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.
next prev parent reply other threads:[~2022-11-01 16:51 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-31 18:10 bug#58929: 29.0.50; Calc: finding roots utpn doesn't work as advertised Matt Armstrong
2022-11-01 16:51 ` Mattias Engdegård [this message]
2022-11-01 22:00 ` Matt Armstrong
2022-11-03 8:53 ` Mattias Engdegård
2022-11-03 9:13 ` Eli Zaretskii
2022-11-05 19:29 ` Matt Armstrong
2022-11-08 11:00 ` Mattias Engdegård
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