FC Tables

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FC Tables is a Fortran 2003 program that computes FC-Gram tables.

This program heavily utilizes the MPFUN2020 library for arbitrary precision floating point arithmetic. This library comes in two variants and both are included. The first one, MPFUN-Fort, is a pure Fortran implementation, which is straightforward to compile. The second variant, MPFUN-MPFR, depends on the C libraries GMP and MPFR and, although trickier to compile, delivers about 3x faster performance. It is, however, very simple to choose which variant of MPFUN should FC Tables try compile.

FC Tables can also take advantage of multicore processors by utilizing OpenMP to split loops across multiple threads.

More information about the FC-Gram algorithm to perform periodic continuations of data can be found at, https://doi.org/10.1016/j.cpc.2020.107482 and https://doi.org/10.1016/j.jcp.2009.11.020, and references therein.

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Compilation

1. Clone/download this repository.
2. cd into the project directory.
3. Edit the text file Makefile.in to set
   1. The desired precision in decimal digits, DIGITS.
   2. Whether to build support for constructing blend-to-zero operators (BLEND = yes) or not (BLEND = no).
   3. The MPFUN variant, fort or mpfr.
   4. Whether OpenMP support should be enabled (OPENMP = yes) or not (OPENMP = no).
   5. Compiler family to use. Options are GNU for gfortran and INTEL for ifort. Note that building with ifort has not been thoroughly tested.
4. Execute the make command.

Note that if MPFUN = mpfr is selected, the first compilation can take up to 20/30 minutes, as mpfr and gmp (its dependency) have very long compilation times. These libraries are only compiled one time unless make distclean is invoked.

Usage

1. Edit parameter.inp and select the desired settings.
2. Run ./fc_tables in the project directory.

To run in N cores, OMP_NUM_THREADS=N ./fc_tables should be used instead.

Runtime options

The file parameter.inp is splitted in three sections, namely required, optional and svd. The optional and svd sections are only used if computing blend-to-zero operators A.

Namelist	Parameter	Default value	Description
required	odir		Directory to write the Q and A arrays.
required	d		Number of matching points to use in the periodic extension. Note that d points generate a d-1 order extension
required	C		Number of continuation points to use in the periodic extension.
required	tkind		=0 to construct Dirichlet projector Q and, if compiled with BLEND=yes, blend-to-zero operator A.=1 to construct Neumann projector. =2 to construct second normal derivative projector.
svd	iters	200	Number of SVD iterations to perform.
svd	resume	0	0 to start a new SVD decomposition, 1 to resume from svd_temp.dat.
svd	sstep	iters+1	Number of SVD iterations between printing partial errors and saving svd_temp.dat for resuming.
svd	eigenmin	1.e-(0.9*DIGITS)	Singular values < eigenmin are considered 0 for numerical stability.
optional	o	20	Oversampling factor for the dense grid.
optional	bw	4	Bandwidth reduction when adjusting a trigonometric polynomial.
optional	z	C/2	Number of gridpoints for the smooth transition to zero.
optional	e	C	Number of gridpoints after the transition to zero region.

Output format

The resulting matrices are saved as double precision raw arrays in Fortran (i.e. column major) order and the endianness is fixed by the CPU architecture.

For the blend-to-zero operator the output filename is AC-d, where C is the number of continuation points and d the number of matching points. Qd is the output filename for the Dirichlet projector.

In the case of projectors involving derivatives (tkind = 1 or tkind = 2), the first element in the output file is the grid spacing and then the appropriate double precision projector array (Qnd or Q2nd, respectively). This grid spacing is useful for using the operators with different grid spacings (via chain rule derivatives).

A sample Python script named load_tables.py showcases how to load each output in a Python environment. The included source code fc_test.f08 shows how to load the operators in a Fortran environment.

Verifying the tables

For convenience a simple Fortran 2008 program that compares numerical (using FC-Gram continuations) to analytical derivatives of the function J_0(x)e^(3*x) is also supplied. This program can verify Dirichlet operators (that is, Q and A), and normal derivative operators (Qn and Q2n). Do note, however, that checking for the derivative operators require the Dirichlet operator for the same number of matching points, that is, to check Qn7.dat, Q7.dat is also required. If a Fortran 2008 compiler isn't available to you, it should be straight forward to edit fc_test.f08, edit the target function (and its derivative) and compile it as a Fortran 2003 program.

To compile the test program simply run, after compiling fc_tables, the command make test and execute the newly generated binary as ./fc_test N, where N the number of grid points to use (which must be a power of 2 due to the simplicity of the fft implementation).

The necessary table is automatically loaded and tested based on the values present in the parameter.inp file.

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Author

Mauro Fontana - Department of Physics - University of Buenos Aires.