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PAPER-CHANGING FINDING: FA is dramatically different from DFA on the
same architecture. FA has genuine deep credit quality where DFA has none.
Implementation:
- experiments/cifar_resmlp.py: added train_fa() + FA diagnostic support
FA uses sequential backward credit propagation with d×d random matrices
(a_l = B_l @ a_{l+1}) instead of DFA's direct output-error projection
(a_l = B_l^T @ e_T). Same local loss form <f_l, a_l>.
Core results (A-H, 100ep 3-seed d=256 terminal-LN ResMLP):
FA main audit: 0.401 ± 0.009 (DFA: 0.306 ± 0.008) +9.5 pp
FA vs frozen: +5.2 pp ABOVE baseline (DFA: -4.3 pp below)
FA deep cos: +0.33 (DFA: ~0 degenerate)
FA ||h_L||: ~10^5 (DFA: ~5×10^8) 3 OOM less growth
FA ||g_L||: ~10^-6 meaningful (DFA: ~10^-10 floor)
Mode 1(b) fires: NO for FA; YES for DFA
FA+pen lam=1e-2: 0.369 ± 0.003 (DFA+pen: 0.360 ± 0.002)
FA+pen lam=1e-4: 0.377 ± 0.006 (DFA+pen lam=1e-4: 0.360)
At lam=1e-4, FA already has deep cos +0.30 while DFA has -0.02
FA random-target: acc 0.12 (chance), h_L=1.3e5 (DFA: 1.7e8)
FA early 5ep: deep cos already +0.32 (DFA ep1: -0.008)
Extension results (d=512 depth sweep, 100ep, s42):
L=2: FA 0.350, cos +0.96 (DFA: n/a)
L=4: FA 0.424, cos +0.29 (DFA: n/a)
L=6: FA 0.401, cos +0.16 (DFA: n/a)
L=8: FA 0.409, cos +0.11 (DFA: 0.306, cos -0.0001)
L=12: FA 0.404, cos +0.09 (DFA: 0.309, cos -0.0001)
FA deep cos is positive at EVERY depth; DFA is ~0 everywhere.
FA accuracy exceeds DFA by 5-10 pp at L=8 and L=12.
This is the strongest empirical support for the Mode 2 → Mode 1
hypothesis: same local loss, same architecture, same optimizer —
only the credit signal differs. FA's sequential propagation produces
much better per-layer credit (cos +0.33 vs ~0), which prevents the
catastrophic activation growth that DFA exhibits.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Patches:
- main(): add --penalty_lam (separate from CB's bridge temperature args.lam)
- train_dfa block update (line 195): add penalty_lam * (f_l**2).sum(-1).mean()
- train_state_bridge block update (line 326): same penalty
- train_credit_bridge block update (line 533): same penalty
Codex round 38 GO STAGE: keep penalty separate from CB lam, blocks-only,
sanity-check that hidden_norms remain nontrivial (not silencing the blocks).
2-epoch smoke (results/round38_smoke_sbcb_pen) passes the silencing check:
SB ||h_L||=229, CB ||h_L||=1258, both nontrivial. Deep cosines positive across
all layers for SB ([0.28, 0.25, 0.23]) and rising for CB ([0.04, 0.08, 0.13, 0.15]).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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- experiments/cifar_resmlp.py: add --methods filter and --random_targets flag;
extend compute_diagnostics to log hidden_norms_per_layer and bp_grad_norms_per_layer
- paper/main.tex §3 ¶1: broaden random-target finding to all 3 fixed-feedback methods
(DFA: ||h_L||=14510, SB: ||h_L||=6225, CB: ||h_L||=19974 at ep 3, all at chance acc)
- paper/main.tex Appendix J: extended with cross-method smoke-test table
This generalizes the §3 mechanism story from 'DFA-specific' to 'all 3 audited
fixed-feedback local-credit methods'. Combined with rounds 32-34, the proximate
cause of Mode 1 (a) is now well-localized:
- Not requires residual skip (round 33 H2 walkback)
- Not requires task signal (round 34 random targets, DFA)
- Not DFA-specific (round 35 random targets, SB+CB)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Debug phase. Toy LQ experiments (3 seeds) complete with terminal gradient matching.
Credit bridge matches state bridge on linear system (~0.94 cosine).
CIFAR experiments in progress.
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