In November 2013 I posted a draft of “Time Varying BIBO Stability Analysis of Trapezoidal integrated optimised SVF v2” to the music-dsp mailing list. The draft applied Jean Laroche’s state-space method  to prove the time varying stability of the SVF filter that Andy Simper had recently derived using trapazoidal integration. I didn’t quite finish the stability proof. You can read the discussion in the mailing list thread to see how close we got.
I recently learnt that Aaron Wishnick had completed the stability proof and had also published a number of useful coefficient formulas for the SVF. Aaron kindly cited my partial proof. Thanks Aaron!
For future reference the maxima source file for my draft analysis is now archived on github.
 Bencina, R. (2013) “Time Varying BIBO Stability Analysis of Trapezoidal integrated optimised SVF v2,” music-dsp mailing list, Nov. 2013, Available at firstname.lastname@example.org/msg02467.html
 Laroche, J. (2007) “On the Stability of Time-Varying Recursive Filters,” J. Audio Eng. Soc., vol. 55, no. 6, pp. 460-471, June 2007.
 Simper, A. (2013) “Solving the Continuous SVF equations Using Trapezoidal Integration and Equivalent Currents,” Nov. 2013, Available at www.cytomic.com/files/dsp/SvfLinearTrapOptimised2.pdf
 Wishnick, A. (2014) “Time-Varying Filters for Musical Applications” Proc. of the 17th Int. Conference on Digital Audio Effects (DAFx-14), Erlangen, Germany, September 1-5, 2014. Available at www.dafx14.fau.de/papers/dafx14_aaron_wishnick_time_varying_filters_fo r_.pdf