.. _main:

``cppdlr``: Imaginary time calculations using the discrete Lehmann representation
=================================================================================

``cppdlr`` is a C++ library implementing the discrete Lehmann representation (DLR) of
imaginary time single-particle Green's functions and other imaginary time
quantities, including standard operations. It can be downloaded `from GitHub <https://github.com/flatironinstitute/cppdlr>`_.

The DLR has been implemented in other programming languages (see the :ref:`related libraries<otherlibraries>` below). 
``cppdlr`` offers a particularly simple high-level user interface, enabled by
the use of C++ templating and the `nda library <https://github.com/TRIQS/nda>`_ for array types and BLAS/LAPACK
compatibility. This facilitates its use both in small-scale applications and in
existing large-scale software projects such as the `TRIQS library
<https://triqs.github.io>`_ for quantum many-body calculations, which has
incorporated the DLR via ``cppdlr``. 

Getting started
---------------

  - Install the library by following the instructions on the :ref:`installation page<install>`.
  - If you are unfamiliar with the DLR, you can read the :ref:`background page<background>` for a
    brief introduction, or the references below for more detail. If you are
    familiar with the DLR, you should still take a brief look at the
    :ref:`background page<background>` for important information about the library.
  - To begin using ``cppdlr`` in your application, the :ref:`examples page<examples>` provides a
    good starting point.
  - If you need more specific information about ``cppdlr`` classes and functions,
    take a look at the :ref:`reference documentation page<documentation>`.

References
----------

If you use ``cppdlr`` in your software or published research works, please mention
this, and cite the following references. Citations help to encourage the
development and maintainence of open-source scientific software.

- The original reference on the DLR: `J. Kaye, K. Chen, O. Parcollet, "Discrete Lehmann representation of imaginary time Green's functions", Phys. Rev. B, 105, 235115 (2022).
  <https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.235115>`_
  [`arXiv:2107.13094 <https://arxiv.org/abs/2107.13094>`_]
- The companion paper to ``cppdlr``: `J. Kaye, H. U. R. Strand, N. Wentzell, "cppdlr: Imaginary time calculations using the discrete Lehmann representation", arXiv:2404.02334 (2024).
  <https://arxiv.org/abs/2404.02334>`_


.. _otherlibraries:

Related libraries
-----------------

Libraries implementing the DLR are available in other languages:

- Python, via `pydlr <https://github.com/jasonkaye/libdlr>`_ 
- Fortran, via `libdlr <https://github.com/jasonkaye/libdlr>`_
- Julia, via `Lehmann.jl <https://github.com/numericaleft/Lehmann.jl>`_

``cppdlr`` is also used to implement the DLR in the `Toolbox for Research on
Interacting Quantum Systems (TRIQS) <https://triqs.github.io/triqs/latest/>`_.
 
.. toctree::
   :maxdepth: 2
   :caption: Contents:

   install.rst
   background.rst
   examples.rst
   documentation.rst
   issues.rst