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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.






  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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