bug #876, matrix_log_compute_2x2: directly use logp1 instead of atanh2

1 files changed, 11 insertions, 6 deletions

diff --git a/unsupported/Eigen/src/MatrixFunctions/MatrixLogarithm.h b/unsupported/Eigen/src/MatrixFunctions/MatrixLogarithm.h
index 42b60b9b1..22bfdc4ac 100644
--- a/unsupported/Eigen/src/MatrixFunctions/MatrixLogarithm.h
+++ b/unsupported/Eigen/src/MatrixFunctions/MatrixLogarithm.h
@@ -53,15 +53,20 @@ void matrix_log_compute_2x2(const MatrixType& A, MatrixType& result)
   result(1,0) = Scalar(0);
   result(1,1) = logA11;
 
-  if (A(0,0) == A(1,1)) {
+  Scalar y = A(1,1) - A(0,0);
+  if (y==Scalar(0))
+  {
     result(0,1) = A(0,1) / A(0,0);
-  } else if ((abs(A(0,0)) < 0.5*abs(A(1,1))) || (abs(A(0,0)) > 2*abs(A(1,1)))) {
-    result(0,1) = A(0,1) * (logA11 - logA00) / (A(1,1) - A(0,0));
-  } else {
+  }
+  else if ((abs(A(0,0)) < 0.5*abs(A(1,1))) || (abs(A(0,0)) > 2*abs(A(1,1))))
+  {
+    result(0,1) = A(0,1) * (logA11 - logA00) / y;
+  }
+  else
+  {
     // computation in previous branch is inaccurate if A(1,1) \approx A(0,0)
     int unwindingNumber = static_cast<int>(ceil((imag(logA11 - logA00) - M_PI) / (2*M_PI)));
-    Scalar y = A(1,1) - A(0,0), x = A(1,1) + A(0,0);
-    result(0,1) = A(0,1) * (Scalar(2) * numext::atanh2(y,x) + Scalar(0,2*M_PI*unwindingNumber)) / y;
+    result(0,1) = A(0,1) * (numext::log1p(y/A(0,0)) + Scalar(0,2*M_PI*unwindingNumber)) / y;
   }
 }