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

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

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14 changes: 14 additions & 0 deletions src/math/all_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -2736,6 +2736,20 @@ func TestJ1(t *testing.T) {
t.Errorf("J1(%g) = %g, want %g", vfj0SC[i], f, j1SC[i])
}
}

// test for https://github.com/golang/go/issues/82101
// J1 is an odd function: J1(-x) = -J1(x).
for _, x := range []float64{SmallestNonzeroFloat64, 1e-30, 1e-9, 1.0 / (1 << 27), 1e-4, 1, 2.5, 1e10} {
if f, g := J1(-x), -J1(x); f != g {
t.Errorf("J1(%g) = %g, want %g", -x, f, g)
}
if f, g := Jn(1, -x), -J1(x); f != g {
t.Errorf("Jn(1, %g) = %g, want %g", -x, f, g)
}
if f, g := Jn(-1, x), -J1(x); f != g {
t.Errorf("Jn(-1, %g) = %g, want %g", x, f, g)
}
}
}

func TestJn(t *testing.T) {
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3 changes: 3 additions & 0 deletions src/math/j1.go
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Original file line number Diff line number Diff line change
Expand Up @@ -132,6 +132,9 @@ func J1(x float64) float64 {
return z
}
if x < TwoM27 { // |x|<2**-27
if sign {
return -0.5 * x
}
return 0.5 * x // inexact if x!=0 necessary
}
z := x * x
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