= Transverse tangent correlation of a stretched worm-like chain
{title2=$C(r)=(k_BT/\sqrt{Af})e^{-|r|/\xi},\quad\xi=\sqrt{A/f}$}
The bulk tangent <covariance> is twice the inverse <Fourier transform> of $k_BT/(Aq^2+f)$, one term for each transverse direction. Equivalently the inverse of $-A\partial_r^2+f$ is $e^{-|r|/\xi}/(2\sqrt{Af})$. This gives the displayed correlation and identifies a force-dependent <correlation length>, different from the zero-force <persistence length>. Finite chains require endpoint conditions or a periodic convention.
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