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Harminv is mostly used via the stand-alone `harminv`

program, but it can also be called as a library from C or C++, as described below.

The usage of the library `-lharminv`

is analogous to the program. In C or C++, you first `#include <harminv.h>`

, then specify the data and the frequency range by calling `harminv_data_create`

, returning a `harminv_data`

data structure:

```
(int n,
harminv_data harminv_data_createconst harminv_complex *signal,
double fmin, double fmax, int nf);
```

Here, `signal`

is a pointer to an array of `n`

complex numbers. In C++, `harminv_complex`

is `std::complex<double>`

. In C, `harminv_complex`

is a `double[2]`

with the real parts in `signal[i][0]`

and the imaginary parts in `signal[i][1]`

. (For a real signal, set the imaginary parts to zero.) `fmin`

and `fmax`

are the frequency range to search, and `nf`

is the number of spectral basis functions (see below). Frequencies are in units corresponding to a sampling interval of 1 time unit; if your actual sampling interval is dt, then you should rescale your frequencies by multiplying them by dt.

A good default for `nf`

is `min(300, (fmax - fmin) * n * 1.1)`

, corresponding to a spectral “density” of at most 1.1 (see also the `-d`

option of the command-line tool). That is, this uses a number of initial basis functions corresponding to the Fourier resolution of `1/n`

. This does *not* determine the frequency resolution of the outputs, which can be much greater than the Fourier resolution. It sets an upper bound on the number of modes to search for, and in some sense is the density with which the bandwidth is initially “searched” for modes. Spectral densities much larger than 1 are not recommended, as they lead to large and singular matrices and unstable results. Note also that the computation time goes as O(n * nf) + O(nf^3).

Then, you solve for the frequencies by calling:

`void harminv_solve(harminv_data d);`

Then, the frequencies and other data can be extracted from `d`

by the following routines. The number N of frequencies found is returned by:

`int harminv_get_num_freqs(harminv_data d);`

Then, for each index 0 <= k < N, the corresponding frequency and decay constant (as defined in `man harminv`

) are returned by:

```
double harminv_get_freq(harminv_data d, int k);
double harminv_get_decay(harminv_data d, int k);
```

Alternative, you can get the complex angular frequency (omega = 2π freq - i decay) by:

`void harminv_get_omega(harminv_complex *omega, harminv_data d, int k); `

You can get the “quality factor” Q (pi |freq| / decay) by:

`double harminv_get_Q(harminv_data d, int k);`

The complex amplitude (|amp| * exp(-I phase)) for each k is returned by:

`void harminv_get_amplitude(harminv_complex *amplitude, harminv_data d, int k);`

A crude estimate of the relative error in the (complex) frequency is:

`double harminv_get_freq_error(harminv_data d, int k);`

As described in `man harminv`

, this is not really an error bar, and should be treated more as a figure of merit (smaller is better).

To link to the library, you need to not only link to `-lharminv`

, but also to the math library, the BLAS and LAPACK libraries (see below), and any libraries that are required to link C with Fortran code (like LAPACK). If you have the `pkg-config`

program installed (standard on most GNU/Linux systems), you can simply do:

```
pkg-config --cflags harminv
pkg-config --libs harminv
```

to get the flags for compiling and linking, respectively. You may need to tell `pkg-config`

where to find `harminv.pc`

if `harminv`

was installed under `/usr/local`

(the default)…in this case, you would specify `/usr/local/lib/pkgconfig/harminv.pc`

instead of `harminv`

above.

There is an additional wrinkle. If you configured harminv with `--with-cxx`

, or if your C compiler did not support C99 complex numbers and the configure script automatically switched to C++, then you will need to link to harminv with the C++ linker, even if your program is written in C, in order to link the C++ libraries.