Use bessel identity to improve grad calc

stan-dev · Jan 20, 2024 · 1dd2dca · 1dd2dca
1 parent c26bd83
commit 1dd2dca
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/stan/math/prim/prob/von_mises_lpdf.hpp b/stan/math/prim/prob/von_mises_lpdf.hpp
@@ -81,7 +81,7 @@ 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(1, kappa_val)
                 / modified_bessel_first_kind(0, kappa_val);
   }