1 files changed, 6 insertions, 0 deletions
diff --git a/utils/m.h b/utils/m.h
index b25248c2..5e45efee 100644
--- a/utils/m.h
+++ b/utils/m.h
@@ -3,6 +3,7 @@
 
 #include <cassert>
 #include <cmath>
+#include <boost/math/special_functions/digamma.hpp>
 
 template <typename F>
 struct M {
@@ -81,6 +82,11 @@ struct M {
     }
   }
 
+  // digamma is the first derivative of the log-gamma function
+  static inline F digamma(const F& x) {
+    return boost::math::digamma(x);
+  }
+
 };
 
 typedef M<double> Md;