Clamp default frequency grid to Nyquist for discrete diskmargin and mu#148
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`sim_diskmargin(L, σ, l, u)` and `structured_singular_value(M0::LTISystem)` both built their default frequency grid up to 1e3 rad/s. For discrete-time inputs whose Nyquist frequency π/Ts is below 1e3, that grid extended beyond Nyquist and `freqresp` aliased (`cis(ω·Ts)` is 2π/Ts-periodic), so the worst-case margin / μ computed on the default grid reflected an aliased frequency rather than the intended one. The MIMO branch of `diskmargin(L, σ; l, u)` is also fixed because it forwards to `sim_diskmargin`. Clamp the upper bound to `min(u, π/L.Ts)` (or `min(3.0, log10(π/M0.Ts))` in log-frequency) when the system is discrete. Continuous-time behavior is unchanged. Users who deliberately want frequencies past Nyquist can still pass an explicit `w` vector. Adds a regression test in `test/test_diskmargin.jl` using `Ts=1.0` (Nyquist ≈ 3.14 rad/s, well below the default 1e3) that confirms the default-grid result agrees with an explicit Nyquist-respecting grid and that the worst-case frequency is at or below Nyquist. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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## master #148 +/- ##
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Summary
sim_diskmargin(L, σ, l, u)andstructured_singular_value(M0::LTISystem)(no-wform) both used a default frequency grid up to 1e3 rad/s. For discrete-time inputs whose Nyquist π/Ts is below 1e3, the grid extended past Nyquist andfreqrespaliased (cis(ω·Ts)is 2π/Ts-periodic), so the worst-case margin / μ from the default grid reflected an aliased frequency rather than the intended one.min(u, π/L.Ts)when the system is discrete. The MIMO branch ofdiskmargin(L, σ; l, u)benefits automatically since it forwards tosim_diskmargin. Continuous-time behavior is unchanged. Users who deliberately want frequencies past Nyquist can still pass an explicitwvector.Test plan
test_diskmargin.jlwithTs=1.0(Nyquist ≈ 3.14 rad/s, well below the default 1e3): asserts the default-grid result agrees with an explicit Nyquist-respecting grid and that the worst-case frequency is at or below Nyquist.Ts=1e-3, Nyquist ≈ 3141 > 1e3) is unaffected because the clamp resolves tomin(1e3, 3141) = 1e3.🤖 Generated with Claude Code