# Twist (mathematics)

In mathematics (differential geometry) twist is the rate of rotation of a smooth ribbon around the space curve $X=X(s)$ , where $s$ is the arc length of $X$ and $U=U(s)$ a unit vector perpendicular at each point to $X$ . Since the ribbon $(X,U)$ has edges $X$ and $X'=X+\varepsilon U$ the twist (or total twist number) $Tw$ measures the average winding of the curve $X'$ around and along the curve $X$ . According to Love (1944) twist is defined by

$Tw={\dfrac {1}{2\pi }}\int \left({\dfrac {dU}{ds}}\times U\right)\cdot {\dfrac {dX}{ds}}ds\;,$ where $dX/ds$ is the unit tangent vector to $X$ . The total twist number $Tw$ can be decomposed (Moffatt & Ricca 1992) into normalized total torsion $T\in [0,1)$ and intrinsic twist $N\in \mathbb {Z}$ as

$Tw={\dfrac {1}{2\pi }}\int \tau \;ds+{\dfrac {\left[\Theta \right]_{X}}{2\pi }}=T+N\;,$ where $\tau =\tau (s)$ is the torsion of the space curve $X$ , and $\left[\Theta \right]_{X}$ denotes the total rotation angle of $U$ along $X$ . Neither $N$ nor $Tw$ are independent of the ribbon field $U$ . Instead, only the normalized torsion $T$ is an invariant of the curve $X$ (Banchoff & White 1975).

When the ribbon is deformed so as to pass through an inflectional state (i.e. $X$ has a point of inflection) torsion becomes singular, but its singularity is integrable (Moffatt & Ricca 1992) and $Tw$ remains continuous. This behavior has many important consequences for energy considerations in many fields of science.

Together with the writhe $Wr$ of $X$ , twist is a geometric quantity that plays an important role in the application of the Călugăreanu–White–Fuller formula $Lk=Wr+Tw$ in topological fluid dynamics (for its close relation to kinetic and magnetic helicity of a vector field), physical knot theory, and structural complexity analysis.