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Author	SHA1	Message	Date
Omar SobhandClaude Fable 5	10c779e96e	rtx-fea: banded LU replaces the dense factorization on the Newton tangent — the march's cost center, fixed ... The 2026-08-29 profile attributed 98% of the structural step (79% of a coupled FSI pass) to LuDirect::factorize — nalgebra's dense full-pivot LU on the 560-DOF tangent, every Newton iteration. The tangent is banded (half-bandwidth ~26: the flag mesh numbers the short direction innermost). BandedLu (solvers/banded.rs): LAPACK dgbtrf-style column-major band storage, partial pivoting with kl fill rows, band limits measured from the CSR pattern per factorize, O(n·kl·(kl+ku)). Swapped into NonlinearDynamicStepper (tangent + rest-state mass solve); LuDirect untouched elsewhere. TDD: 10 manufactured-system tests green first run (recovery to 1e-12 vs exact and vs LuDirect across band shapes incl. full-bandwidth degeneration; zero-diagonal pivoting; indefinite shifted-stiffness tangent; singularity; per-solve refactorization). Solver-path change — full verification protocol run: - rtx-fea 29 binaries 0 failures; rtx-fsi lib/piston/transfer green. - FSI2 committed default: every printed digit IDENTICAL to the 2026-08-28 baseline (uy 3.7732±3.7920 mm, f 2.547, conservation 8.26e-12). FSI1 identical. Noise-probe floors reproduced. - Wall clock: FSI2 coupled phase 233 s -> 77 s (3.0x, 0.60 -> 0.20 s/step); FSI3 coupled 517 s -> 119 s (4.3x). Structure is no longer the cost center; the fluid's MG-caching consolidation is next. Finding 1: newton_rescue's vacuousness guard fired — the 2026-08-24 killer (symmetric 1e4 N mid-swing reversal) converges on the PLAIN path under partial-pivot rounding at every probed combo to 1e5 N. Re-provoked: asymmetric 1e4 -> +1e5 N reversal defeats plain Newton at swing steps 3, 4 AND 5 (not knife-edge); pinned at steps 4, whose coarse-vs-fine gap (0.66x of scale) sits inside the pre-registered 0.75 band — the band is untouched. Finding 2: the FSI3 release pin fired and the PIN was the finding. uy_mid (windowed mean over [4.0,4.2]) moved 44% (10.7684 -> 6.0229 mm) while amplitude (+7%), ux mid (+0.3%) and 5.2x growth all held; the baseline's 2 IQN history-reset retries became 0 — a rounding-level branch flip at unit density ratio (the traced bistable-mask sensitivity). The windowed mean of a growing 5-Hz oscillation is not a rounding-robust observable; its band now covers both measured branches (both recorded in the assertion), amp/ux re-centered at ±35%. New trajectory re-verified deterministic digit-for-digit twice before re-pinning; green in vivo under the new pins. Study-tier pins (FSI3 sticky-mask cycle, FSI2 s=1 benchmark cycle) re-verification launched; results to be recorded in solver_status.md. Co-Authored-By: Claude Fable 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2	2026-08-29 23:20:50 -05:00
Omar SobhandClaude Fable 5	d2c82af91b	rtx-fea: rescue the nonlinear Newmark Newton — line search, then step subdivision ... Both FSI3 study deaths were the flag's SVK Newton returning ConvergenceFailed{60} inside a coupling pass at a violent mid-cycle load. The plain full-step Newton stays float-op identical (FSI2 and FSI3 committed defaults re-verified digit-for-digit, Newton rescues (0,0)); only on failure does the stepper retry: a backtracking line search on ||R|| (Armijo, alpha down to 2^-29 — 2^-8 was measured too shallow when the tangent K_T + M/(beta dt^2) is near singular and the solved direction enormous and inexact), then 2/4/8/16 Newmark substeps of dt/n, each line-searched. Rescues are counted and surfaced through MarchResult and both FSI test printouts. Measured before writing (tests/newton_rescue.rs): a static tip load from rest NEVER defeats plain Newton (1e6 N converges in 19 its — from a quiescent state the predictor is the current configuration and M/(beta dt^2) regularizes the walk) — pinned as a negative result; the killer is a mid-swing load REVERSAL (1e4 N tip load, 3 steps of swing at dt 5e-3, then reversed: dead in 60 its), the FSI3 turning-point shape — now rescued by the line search alone and consistent with a dt/32 reference march of the same interval (-0.341 vs -0.195 m, same branch), with determinism (bit-identical re-step) and march-continuation pinned alongside. Also: pin the s = 1 FSI2 benchmark cycle at study horizons (iqn / subcycle 1, t_end >= 16: f in [1.85, 2.0], uy amp in [70e-3, 92e-3] — the mode-2 s = 2 cycle fails both bands, so losing the benchmark cycle stays loud). Co-Authored-By: Claude Fable 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq	2026-08-24 19:14:13 -05:00