[python-control-developers] [python-control/python-control] 460812: Add scipy fallbacks for generalized Lyapunov and d...

[Scott C. L. <no...@gi...>]

  Branch: refs/heads/main
  Home:   https://github.com/python-control/python-control
  Commit: 46081238fa775efcaf67816379112cf4b711dc0d
      https://github.com/python-control/python-control/commit/46081238fa775efcaf67816379112cf4b711dc0d
  Author: Kangwon Lee <219...@us...>
  Date:   2026-08-13 (Thu, 13 Aug 2026)

  Changed paths:
    M control/mateqn.py
    M control/tests/mateqn_test.py

  Log Message:
  -----------
  Add scipy fallbacks for generalized Lyapunov and discrete Sylvester equations

lyap and dlyap previously raised ControlArgument for method='scipy'
(explicit, or auto-selected when slycot is absent) on three cases that
SLICOT handles. Add pure scipy/numpy fallbacks for all three:

- Generalized Lyapunov, continuous and discrete (E != I): congruence
  transform by inv(E) to a standard Lyapunov equation, then scipy's
  solve_continuous/discrete_lyapunov. Requires E nonsingular; SLICOT
  sg03ad (Penzl's generalized Schur method) also handles singular E and
  remains the method='slycot' path. A nonsingular-E failure raises a
  clear ControlArgument. Because the transform inverts E, an
  ill-conditioned E yields reduced accuracy; this now emits a UserWarning
  recommending method='slycot' (the continuous path was previously
  silent, while the discrete path warned only incidentally).

- Discrete Sylvester (A X Q^T - X + C = 0): Bartels-Stewart method via
  complex Schur factors of A and Q^T with column-by-column triangular
  solves, O(n^3 + m^3), matching the Hessenberg-Schur cost of SLICOT
  sb04qd. Includes the solvability check that no eigenvalue pair of
  A and Q^T has product (almost) equal to 1.

Also fix two incorrect slycot import-fallback aliases (sb0qmd -> sb04qd,
sb04ad -> sg03ad) so the names resolve to None, not NameError, when
slycot is absent; and correct the dlyap Notes, which described the
scipy Sylvester path as a Kronecker O((nm)^3) solve (it is now
Bartels-Stewart, O(n^3 + m^3)).

Tests parametrize method=[None, 'scipy', 'slycot'] and cross-check that
the scipy and slycot solutions agree, and cover the nonsingular-E
requirement, the ill-conditioned-E warning, and the singular
discrete-Sylvester case.

Co-Authored-By: Claude Fable 5 <no...@an...>
Co-Authored-By: Claude Opus 4.8 <no...@an...>


  Commit: 89b2f686c1d504c8fb0c480d629e143514883f65
      https://github.com/python-control/python-control/commit/89b2f686c1d504c8fb0c480d629e143514883f65
  Author: Kangwon Lee <219...@us...>
  Date:   2026-08-13 (Thu, 13 Aug 2026)

  Changed paths:
    M control/mateqn.py

  Log Message:
  -----------
  Correct SG03AD singular-E claim in lyap/dlyap docs

The generalized-Lyapunov Notes, the scipy-path comments, and the
nonsingular-E error messages stated that SLICOT SG03AD "also handles
singular E". It does not: SG03AD factors the matrix pencil without
inverting E (its advantage for a nonsingular but ill-conditioned E),
but a truly singular (descriptor) E returns a degenerate-pair warning
and is out of scope for both the scipy and slycot paths. Correct all
six spots (continuous lyap and discrete dlyap) to say so.

Co-Authored-By: Claude Opus 4.8 <no...@an...>


  Commit: 3f6dbac2a4793b8bad535cfe16321acaf72f35eb
      https://github.com/python-control/python-control/commit/3f6dbac2a4793b8bad535cfe16321acaf72f35eb
  Author: Kangwon Lee <219...@us...>
  Date:   2026-08-13 (Thu, 13 Aug 2026)

  Changed paths:
    M control/mateqn.py

  Log Message:
  -----------
  Correct ill-conditioned-E accuracy claim in lyap/dlyap

The generalized-Lyapunov Notes and the _warn_ill_conditioned_E helper
(its docstring and warning message) claimed method='slycot' is
"preferable" / "more robust" than method='scipy' for ill-conditioned E.
Benchmarking (scipy vs slycot, n=100, cond(E) swept 1 to 1e12 over 5
seeds) shows both methods lose accuracy at the same rate -- slycot is in
fact 1.3-2x *less* accurate for cond(E) >= 1e4, never better. The
generalized Lyapunov problem is itself ill-conditioned (about cond(E)^2)
when E is, so no method beats that floor. Reword to say both degrade and
slycot is not measurably more accurate, and drop the false "use
method='slycot' for a more robust solution" line from the warning.

Co-Authored-By: Claude Opus 4.8 <no...@an...>


  Commit: 2dabe33ccd4b9b9f3cf8fc54837fc183b09b86a4
      https://github.com/python-control/python-control/commit/2dabe33ccd4b9b9f3cf8fc54837fc183b09b86a4
  Author: Kangwon Lee <219...@us...>
  Date:   2026-08-13 (Thu, 13 Aug 2026)

  Changed paths:
    A benchmarks/scipy_fallback_bench.py

  Log Message:
  -----------
  Add benchmarks for scipy vs slycot matrix-equation fallbacks

Add benchmarks/scipy_fallback_bench.py (asv) comparing the pure-scipy
fallbacks with slycot for generalized Lyapunov (continuous and discrete)
and discrete Sylvester, plus an accuracy-vs-cond(E) track for the
generalized Lyapunov solution. slycot parameterizations are skipped when
slycot is not installed. Requested in the PR #1234 review.

Co-Authored-By: Claude Opus 4.8 <no...@an...>


  Commit: a7754794dcc9d3ef2060cc1b2486bd42b73395cc
      https://github.com/python-control/python-control/commit/a7754794dcc9d3ef2060cc1b2486bd42b73395cc
  Author: Scott C. Livingston <sli...@cd...>
  Date:   2026-08-13 (Thu, 13 Aug 2026)

  Changed paths:
    A benchmarks/scipy_fallback_bench.py
    M control/mateqn.py
    M control/tests/mateqn_test.py

  Log Message:
  -----------
  Merge pull request #1234 from @kangwonlee

Add scipy fallbacks for generalized Lyapunov and discrete Sylvester equations


Compare: https://github.com/python-control/python-control/compare/c7ea9c92915c...a7754794dcc9

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