The special functions module contains functions that are based on the
C implementation in the Cephes library (http://www.netlib.org/cephes/).
They are considered a derivative of the Cephes Math Library that is compatibly licensed:

Name: Cephes
Files: src/special_functions.rs
License: 3-clause BSD
  Distributed under 3-clause BSD license with permission from the author,
  see https://lists.debian.org/debian-legal/2004/12/msg00295.html

  Cephes Math Library Release 2.8:  June, 2000
  Copyright 1984, 1995, 2000 by Stephen L. Moshier

  This software is derived from the Cephes Math Library and is
  incorporated herein by permission of the author.

  All rights reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:
      * Redistributions of source code must retain the above copyright
        notice, this list of conditions and the following disclaimer.
      * Redistributions in binary form must reproduce the above copyright
        notice, this list of conditions and the following disclaimer in the
        documentation and/or other materials provided with the distribution.
      * Neither the name of the <organization> nor the
        names of its contributors may be used to endorse or promote products
        derived from this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
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