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Fix case where b is a negative integer and z is also negative. (#983)

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* Fix case where b is a negative integer and z is also negative.
Add tests etc.
Fixes: https://github.com/boostorg/math/issues/982.
This commit is contained in:
jzmaddock
2023-05-04 09:17:16 +01:00
committed by GitHub
parent 298a243ccd
commit d5960de3db
5 changed files with 157 additions and 7 deletions
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29
include/boost/math/special_functions/hypergeometric_1F1.hpp
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@@ -460,13 +460,28 @@ namespace boost { namespace math { namespace detail {
return hypergeometric_1F1_checked_series_impl(a, b, z, pol, log_scaling);
}
}
// Let's otherwise make z positive (almost always)
// by Kummer's transformation
// (we also don't transform if z belongs to [-1,0])
long long scaling = lltrunc(z);
T r = exp(z - scaling) * detail::hypergeometric_1F1_imp<T>(b_minus_a, b, -z, pol, log_scaling);
log_scaling += scaling;
return r;
if ((b < 0) && (floor(b) == b))
{
// Negative integer b, so a must be a negative integer too.
// Kummer's transformation fails here!
if(a > -50)
return detail::hypergeometric_1F1_generic_series(a, b, z, pol, log_scaling, function);
// Is there anything better than this??
return hypergeometric_1F1_imp(a, float_next(b), z, pol, log_scaling);
}
else
{
// Let's otherwise make z positive (almost always)
// by Kummer's transformation
// (we also don't transform if z belongs to [-1,0])
// Also note that Kummer's transformation fails when b is
// a negative integer, although this seems to be unmentioned
// in the literature...
long long scaling = lltrunc(z);
T r = exp(z - scaling) * detail::hypergeometric_1F1_imp<T>(b_minus_a, b, -z, pol, log_scaling);
log_scaling += scaling;
return r;
}
}
//
// Check for initial divergence: