inst/trefoil_circles.R

In knotR: Knot Diagrams using Bezier Curves

## This R script plots (a projection of) a trefoil knot assuming that
## the strand is an elastica.  The constraints are: (1), strands cross
## at right angles; the strands exert a couple but no force on one
## another; (2), mirror symmetry about the vertical; (3), threefold
## rotational symmetry.  Constraint #1 implies that the strands are
## arcs of a circle.

## Elementary trigonometry then shows that the radii of the inner and
## outer loops are cosec(pi/12) and sec(pi/12) = sqrt(6)+sqrt(2) and
## sqrt(6)-sqrt(2) respectively.

## NB this file produces trefoil_circles.pdf, which is not production
## quality!  It needs to be tidied up in inkscape before anyone sees
## it.

library("knotR")

`arc` <- function(x,y,r,theta1,theta2,rot=0,center=FALSE, ...){
  n <- 100
  jj <- matrix(c(cos(rot),sin(rot),-sin(rot),cos(rot)),2,2) %*% c(x,y)
  x <- jj[1]
  y <- jj[2] 
  if(center){ points(x,y, ...) } 
  theta <- seq(from=theta1-rot,to=theta2-rot,len=n)
  points(x+r*sin(theta),y+r*cos(theta),type='l',...)
}

small <- 0.05

`f` <- function(angle,...){
  arc(x=0, y=-2-2/sqrt(3), theta1 =   0, theta2 =   pi/12, r=sqrt(6)+sqrt(2), rot=angle,col='red',lwd=17,...)
  arc(x=0, y=+2-2/sqrt(3), theta1 =  0, theta2 = 7*pi/12, r=sqrt(6)-sqrt(2), rot=angle,col='red',lwd=17,...)

  arc(x=0, y=-2-2/sqrt(3), theta1 = pi/12-small, theta2 =   pi/12+small, r=sqrt(6)+sqrt(2), rot=angle,col='blue',lwd=39,...)

  arc(x=0, y=-2-2/sqrt(3), theta1 =    -pi/12, theta2 =   0, r=sqrt(6)+sqrt(2), rot=angle,lwd=17,...)
  arc(x=0, y=+2-2/sqrt(3), theta1 =  -7*pi/12, theta2 = 0, r=sqrt(6)-sqrt(2), rot=angle,lwd=17,...)
}

pdf(file="trefoil_circles.pdf",height=9,width=9)
par(pty='s')
u <- 1.9
plot(NA,type='n',asp=1,xlim=c(-u,u),ylim=c(-u,u),axes=FALSE,xlab='',ylab='')

f(0*pi/3)
f(2*pi/3)
f(4*pi/3)

dev.off()


pdf(file="trefoil_proper.pdf",height=9,width=9)
knotplot(k3_1,lwd=16)
dev.off()