add `hypergeometric_1F1`, `log_hypergeometric_1F1` and their gradients
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- Dominant language
- C++
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Description
Boost has both of these functions.
The hypergeometric 1F1 function is interesting in that the lower incomplete gamma function can be expressed as
$$
\gamma(a, x) = a^{-1} x^a { _1F_1(a, 1 + a, -x)}
$$
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by locating the existing special-function implementations and gradient tests in the Stan Math library, then compare their conventions with Boost's hypergeometric 1F1 and log-hypergeometric 1F1 functions. Done means both functions are available with forward, reverse, and other required gradients, with tests covering their values and derivatives.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- backend
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100