added SpM paper

doc/about.rst

@@ -77,6 +77,10 @@ In addition to the above two libraries, you may use impurity solvers listed belo
 
 For some of them, they provide a BibTeX entry for each paper in the above cites.
 
+When you use the SpM method for analytic continuation, please cite the following paper:
+
+* `Junya Otsuki, Masayuki Ohzeki, Hiroshi Shinaoka, and Kazuyoshi Yoshimi, Phys. Rev. E 95, 061302(R) (2017) <https://journals.aps.org/pre/abstract/10.1103/PhysRevE.95.061302>`_
+
 GitHub repository
 -----------------
 

doc/analytic_continuation/spm.rst

@@ -9,7 +9,7 @@ The sparse-modeling (SpM) method is a method for the analytic continuation of th
   
 where :math:`\Sigma(\tau)` is the self-energy in imaginary time and :math:`\Sigma(\omega)` is the self-energy in real frequency.
 
-In the SpM method, the kernel matrix of the integral equation, :math:`e^{-\tau\omega}/(1+e^{-\beta\omega})` is decomposed by the singular value decomposition (SVD),
+In `the SpM method <https://journals.aps.org/pre/abstract/10.1103/PhysRevE.95.061302>`_, the kernel matrix of the integral equation, :math:`e^{-\tau\omega}/(1+e^{-\beta\omega})` is decomposed by the singular value decomposition (SVD),
 and the self-energies :math:`\Sigma(\tau)` and :math:`\Sigma(\omega)` are transformed by the left and right singular vectors, respectively.
 
 The SpM method is implemented in the ``dcore_anacont`` program.