Linking Number

A C++ class with MATLAB interface for the computation of the linking number. Given two parametric closed curves $\mathbf{p}(t)$ and $\mathbf{q}(t)$ where

$$ \begin{array}{rcl} [0,N] & \to & \mathbb{R}^3 \cr t & \to & \mathbf{p}(t) \end{array} \qquad \begin{array}{rcl} [0,M] & \to & \mathbb{R}^3 \cr t & \to & \mathbf{q}(t) \end{array} $$

and $\mathbf{p}(0)=\mathbf{p}(N)$, $\mathbf{q}(0)=\mathbf{q}(M)$. The linking number $L(\mathbf{p},\mathbf{q})$ of the two curve $\mathbf{q}(t)$ and $\mathbf{q}(t)$ is defined as the double integral

$$ L(\mathbf{p},\mathbf{q})= \frac{1}{4\pi} \int_{0}^N \int_{0}^M \frac{(\mathbf{q}(s)-\mathbf{p}(t))\dot(\mathbf{q}'(s)\times\mathbf{p}'(t))} {||\mathbf{q}(s)-\mathbf{p}(t)||^3} \mathrm{d}s\mathrm{d}t $$

which can be proved its an integer (if the curve do not intersect).

The library can compute the linking number of two piecewise linear curves where the curves are defined by the support points

$$ {\mathbf{p}_0,\mathbf{p}_1,\ldots,\mathbf{p}_N} \qquad {\mathbf{q}_0,\mathbf{q}_1,\ldots,\mathbf{q}_M} $$

C++ interface

Load the library

#include "linking_number.hh"

Instantiate the liking number manager:

unsigned max_curve_stored = 2;
LK_class lk(max_curve_stored);

the class lk can store more than two curves. Now insert the curves by points, incrementally

unsigned ncurve=0;
lk.reset( ncurve ); // initialize curve

lk.add_point( ncurve, x0, y0, z0 );
lk.add_point( ncurve, x1, y1, z1 );

// ...

lk.add_point( ncurve, xn, yn, zn );
lk.close_curve( ncurve );

and compute linking number

unsigned ncurve0=0;
unsigned ncurve1=1;

int lknumber = lk.eval(ncurve0,ncurve1);

A curve can be inserted by passing all the points stored in a vector

std::vector<Tvec> a;

// ... fill the vector

lk.add_curve( ncurve, pnts );

where Tvec is any structure or class such that the field x, y and z are available. Another possibility is to use a n x 3 matrix as follows

double curve[npts][3];

// fill the points

lk.add_curve( ncurve, curve, npts );

linking number can be computed in one shot passing two vectors or two matrices n x 3 as follows

// mode 1

std::vector<Tvec> a;
std::vector<Tvec> b;

// ... fill the vectors

int lknumber = lk.eval( a, b );

// mode 2

double curve1[10000][3];
double curve2[10000][3];

unsigned nseg1 = xx; // number of segment of the first curve
unsigned nseg2 = xx; // number of segment of the second curve

// fill the points

int lknumber = lk.eval( curve1, nseg, curve2, nseg );

Having loaded m curves it is possible to compute directly a matrix of linking number where Mij is the linking number of the i-th curve vs j-th curve. For example

unsigned i_curve[] = {2,0};
unsigned j_curve[] = {0,2,1};

unsigned ni = 2;
unsigned nj = 3;

int mat[6];

lk.evals( i_curve, ni, j_curve, nj, mat );

and matrix mat contains (using Fortran addressing mat(i,j) = mat[i+2*j]). the following linking numbers:

$L(c_2,c_0)$	$L(c_2,c_2)$	$L(c_2,c_1)$
$L(c_0,c_0)$	$L(c_0,c_2)$	$L(c_0,c_1)$

MATLAB interface

Matlab usage is very easy, there is a unique command lk with a variable number of arguments lk(p1,p2,...,pn) where pk is a n x 3 matrix storing the points of the piecewise linear curve.

[L,err] = lk(p1,p2,...,pn);

L	Linking number matrix
L(i,j)	Linking number of curve i vs curve j, L(i,i) = 0
err	linking number error matrix
err(i,j)	if abs(err(i,j)) < 0.5 the L(i,j) is certified

Installation

Download the toolbox from https://github.com/ebertolazzi/LinkingNumber/releases and install as usual. After installation compile the mex interface by running CompileLK on the MATAB command window.

Citation

If you use this work please cite one or more work in the reference sections. To cite specifically the software use the of Zenodo service.

References

The algorithm used in the library is detailed in

• Enrico Bertolazzi, Riccardo Ghiloni, Ruben Specogna, Efficient computation of Linking number with certification, 2019, link at arxiv https://arxiv.org/abs/1912.13121

This implementation of linking number was used in a various paper here listed

• Ana Alonso Rodríguez, Enrico Bertolazzi, Alberto Valli, The curl-div system: theory and finite element approximation, chapter of the book: Maxwell’s Equations: Analysis and Numerics, de Gruyter., 20-19

• Ana Alonso Rodríguez, Enrico Bertolazzi, Riccardo Ghiloni, Ruben Specogna, Efficient construction of 2-chains representing a basis of $H_2(\Omega^-,\partial\Omega;\mathbb{Z})$, Advances in Computational Mathematics, vol.44, n.5, 2018

• Ana Alonso Rodríguez, Enrico Bertolazzi, Riccardo Ghiloni, Ruben Specogna, Efficient construction of 2-chains with a prescribed boundary, SIAM Journal on Numerical Analysis, vol.55, n.3, 2017

• Ana Alonso Rodriguez, Enrico Bertolazzi, Riccardo Ghiloni, Alberto Valli, Finite element simulation of eddy current problems using magnetic scalar potentials, Journal of Computational Physics, vol. 294, 2015

• Ana Alonso Rodríguez, Enrico Bertolazzi, Riccardo Ghiloni, Alberto Valli, Construction of a finite element basis of the first de Rham cohomology group and numerical solution of 3D magnetostatic problems, SIAM Journal on Numerical Analysis, vol.51, N.4, 2013