Merge pull request #5224 from larsmans/hessinv-doc

DOC: optimize: type of OptimizeResult.hess_inv varies

scipy/optimize/lbfgsb.py

@@ -372,6 +372,9 @@ class LbfgsInvHessProduct(LinearOperator):
     memory approximation to the inverse Hessian, accumulated during the
     optimization.
 
+    Objects of this class implement the ``scipy.sparse.linalg.LinearOperator``
+    interface.
+
     Parameters
     ----------
     sk : array_like, shape=(n_corr, n)

scipy/optimize/optimize.py

@@ -86,10 +86,14 @@ class OptimizeResult(dict):
         underlying solver. Refer to `message` for details.
     message : str
         Description of the cause of the termination.
-    fun, jac, hess, hess_inv : ndarray
-        Values of objective function, Jacobian, Hessian or its inverse (if
+    fun, jac, hess: ndarray
+        Values of objective function, its Jacobian and its Hessian (if
         available). The Hessians may be approximations, see the documentation
         of the function in question.
+    hess_inv : object
+        Inverse of the objective function's Hessian; may be an approximation.
+        Not available for all solvers. The type of this attribute may be
+        either np.ndarray or scipy.sparse.linalg.LinearOperator.
     nfev, njev, nhev : int
         Number of evaluations of the objective functions and of its
         Jacobian and Hessian.