TOMS743
Evaluation of Lambert's W function.

TOMS743 is a FORTRAN77 library which evaluates Lambert's W function. This is a version of ACM TOMS algorithm 743, by Barry, Barry and Culligan-Hensley.

Lambert's W function W(X) satisfies the equation

        W(x) * exp ( W(x) ) = x
      

The text of many ACM TOMS algorithms is available online through ACM: http://www.acm.org/pubs/calgo or NETLIB: http://www.netlib.org/toms/index.html.

Licensing:

The computer code and data files made available on this web page are distributed under the GNU LGPL license.

Languages:

TOMS743 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.

Related Data and Programs:

TEST_VALUES, a FORTRAN77 library which contains routines which return sample values of various functions, including the modified beta function, and the logarithm of the gamma function.

TOMS443, a FORTRAN77 library which evaluates Lambert's W function. This is a version of ACM TOMS algorithm 443.

Reference:

1. Fred Fritsch, RE Shafer, WP Crowley,
   Algorithm 443: Solution of the Transcendental Equation W*exp(W)=X,
   Communications of the ACM,
   Volume 16, Number 1, February 1973, pages 123-124.
2. Andrew Barry, S. J. Barry, Patricia Culligan-Hensley,
   Algorithm 743: WAPR - A Fortran routine for calculating real values of the W-function,
   ACM Transactions on Mathematical Software,
   Volume 21, Number 2, June 1995, pages 172-181.

Source Code:

• toms743.f, the source code.
• toms743.sh, BASH commands to compile the source code.

Examples and Tests:

• toms743_prb.f, a sample calling program.
• toms743_prb.sh, BASH commands to compile and run the sample program.
• toms743_prb_output.txt, the output file.

List of Routines:

• BISECT approximates the W function using bisection.
• CRUDE returns a crude approximation for the W function.
• NBITS_COMPUTE computes the mantissa length minus one.
• TIMESTAMP prints out the current YMDHMS date as a timestamp.
• WAPR approximates the W function.

You can go up one level to the FORTRAN77 source codes.