[Maxima-bugs] [ maxima-Bugs-3587184 ] erf inaccurate for small float values

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Bugs item #3587184, was opened at 2012-11-14 10:19
Message generated for change (Comment added) made by rtoy
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Category: Lisp Core - Floating point
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Private: No
Submitted By: Raymond Toy (rtoy)
Assigned to: Nobody/Anonymous (nobody)
Summary: erf inaccurate for small float values

Initial Comment:
erf(1d-10) -> 1.128e-40

But the taylor series says erf(x) = 2*x/sqrt(%pi) so erf(1d-10) should be 1.128e-20.

The error is in slatec:derf which is using the wrong formula when x < sqeps.

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>Comment By: Raymond Toy (rtoy)
Date: 2012-11-14 19:34

Message:
Fixed by modifying derf.f and derf.lisp

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Comment By: Raymond Toy (rtoy)
Date: 2012-11-14 10:20

Message:
Change summary to mention this is just for float values.

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You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=104933&aid=3587184&group_id=4933