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Remove incorrect logistic (sigmoid) convexity rule #124

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Merged
ChrisRackauckas merged 1 commit into from
Jul 12, 2026
Merged

Remove incorrect logistic (sigmoid) convexity rule #124

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1 change: 0 additions & 1 deletion docs/src/atoms.md
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Expand Up @@ -47,7 +47,6 @@ This page is intended to be a reference for the atoms that are currently impleme
| kldivergence | (array_domain(HalfLine{Real,:open}, 1), array_domain(HalfLine{Real,:open}, 1)) | Positive | Convex | AnyMono |
| lognormcdf | ℝ | Negative | Concave | Increasing |
| log1p | Interval{:open,:open}(-1, Inf) | Negative | Concave | Increasing |
| logistic | ℝ | Positive | Convex | Increasing |
| max | (ℝ, ℝ) | AnySign | Convex | Increasing |
| min | (ℝ, ℝ) | AnySign | Concave | Increasing |
| ^(x, i) | See below | See below | See below | See below |
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2 changes: 0 additions & 2 deletions src/atoms.jl
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Expand Up @@ -482,8 +482,6 @@ add_dcprule(lognormcdf, RealLine(), Negative, Concave, Increasing)

add_dcprule(log1p, Interval{:open, :open}(-1, Inf), Negative, Concave, Increasing)

add_dcprule(logistic, RealLine(), Positive, Convex, Increasing)

add_dcprule(max, (RealLine(), RealLine()), AnySign, Convex, Increasing)
add_dcprule(min, (RealLine(), RealLine()), AnySign, Concave, Increasing)

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10 changes: 10 additions & 0 deletions test/test.jl
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Expand Up @@ -178,3 +178,13 @@ ex = propagate_curvature(propagate_sign(ex))
ex = maximum(exp.(z)) |> unwrap
ex = propagate_curvature(propagate_sign(ex))
@test getcurvature(ex) == SymbolicAnalysis.Convex

# logistic (the sigmoid 1/(1+exp(-x))) is NOT globally convex: f'' is positive
# for x<0, zero at x=0, negative for x>0 (inflection at 0), so no single-curvature
# DCP rule is valid. The rule was dead today (logistic expands to `/`) but a latent
# trap: it must carry no rule and analyze as UnknownCurvature, not a false Convex.
@variables x
@test !SymbolicAnalysis.hasdcprule(logistic)
ex = logistic(x) |> unwrap
ex = propagate_curvature(propagate_sign(ex))
@test getcurvature(ex) == SymbolicAnalysis.UnknownCurvature
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