fixes #3035 by using log_modified_bessel to improve stability

stan-dev · Mar 23, 2024 · 787256c · 787256c
1 parent e8ab249
commit 787256c
Showing 1 changed file with 2 additions and 2 deletions.
diff --git a/stan/math/prim/prob/von_mises_lpdf.hpp b/stan/math/prim/prob/von_mises_lpdf.hpp
@@ -81,8 +81,8 @@ return_type_t<T_y, T_loc, T_scale> von_mises_lpdf(T_y const& y, T_loc const& mu,
   if (!is_constant_all<T_scale>::value) {
     edge<2>(ops_partials).partials_
         = cos_mu_minus_y
-          - modified_bessel_first_kind(1, kappa_val)
-                / modified_bessel_first_kind(0, kappa_val);
+          - exp(log_modified_bessel_first_kind(1, kappa_val)
+                - log_modified_bessel_first_kind(0, kappa_val));   
   }
 
   return ops_partials.build(logp);