Use Eigen's `igamma_der_a` for the incomplete gamma gradient roots - #3408

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avehtari wants to merge 5 commits into

stable-inc-gamma

Commits on Sep 18, 2026

Use Eigen igamma_der_a for the incomplete gamma gradient roots
avehtari
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b172d61

Commits on Sep 19, 2026

Remove the duplicated incomplete gamma series from fwd gamma_q
avehtari
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af3205c

Commits on Sep 20, 2026

Evaluate the fwd gamma_q second-argument derivative in log space
avehtari
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f23eaea
Test the forward-mode gamma_q derivatives at large z and large a
avehtari
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1a9df72
Keep code comments descriptive; move the rationale to the PR text
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ad3ba69