Determinant of helical resistance in resistive-force theory
= Determinant of helical resistance in resistive-force theory
{title2=$AD-B^2=\ell^2a^2\xi_\parallel\xi_\perp$}
Local <resistive-force theory> for a <helix> gives $AD-B^2=\ell^2a^2\xi_\parallel\xi_\perp$. Positive local drag therefore guarantees a positive-definite axial resistance matrix and positive viscous power loss. The identity also keeps the denominator in counterrotating two-helix balances positive.