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math: fix sign of J1 for small negative arguments - #82103

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shogo82148:fix-j1-sign-82101
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math: fix sign of J1 for small negative arguments#82103
shogo82148 wants to merge 1 commit into
golang:masterfrom
shogo82148:fix-j1-sign-82101

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@shogo82148 shogo82148 commented Oct 9, 2026 •
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J1 records the sign of x and negates x so that it is positive,
then restores the sign before returning.
However, the branch for |x| < 2**-27 returned 0.5*x
without restoring the sign, so J1(x) returned a positive value
for -2**-27 < x < 0.
Jn(1, x) and Jn(-1, x) call J1 and were affected as well.

Restore the sign in that branch,
and add a test that J1, Jn(1, x) and Jn(-1, x) are odd.

Fixes #82101

@shogo82148
math: fix sign of J1 for small negative arguments
5963b0c 
J1 records the sign of x and negates x so that it is positive,
then restores the sign before returning. However, the branch for
|x| < 2**-27 returned 0.5*x without restoring the sign, so J1(x)
returned a positive value for -2**-27 < x < 0. Jn(1, x) and
Jn(-1, x) call J1 and were affected as well.

Restore the sign in that branch, and add a test that J1, Jn(1, x)
and Jn(-1, x) are odd.

Fixes golang#82101
@gopherbot

gopherbot commented Oct 9, 2026

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This PR (HEAD: 5963b0c) has been imported to Gerrit for code review.

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@gopherbot

gopherbot commented Oct 9, 2026

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

Please don’t reply on this GitHub thread. Visit golang.org/cl/847626.
After addressing review feedback, remember to publish your drafts!

@gopherbot

gopherbot commented Oct 9, 2026

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Message from 一野瀬翔吾:

Patch Set 1:

(2 comments)

Please don’t reply on this GitHub thread. Visit golang.org/cl/847626.
After addressing review feedback, remember to publish your drafts!

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math: J1 returns the wrong sign for small negative arguments

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