LocalLearningCoefficient

Local Learning Coefficient (LLC) report

Demonstrates TNNet.LocalLearningCoefficientReport, an empirical estimate of the Local Learning Coefficient (LLC) — the Real Log Canonical Threshold (RLCT) from Singular Learning Theory (Watanabe; Lau, Murfet, Wei et al. 2023, "Quantifying Degeneracy in Singular Models via the LLC").

The LLC measures the volume-scaling / effective dimensionality of the minimum the optimizer settled into. It counts the flat, degenerate directions a second-order Hessian top-eigenvalue cannot see: a redundant / over-parameterised solution has LLC_hat << dim(w) — far fewer effective degrees of freedom than raw weights.

Estimator (SGLD-based WBIC / free-energy form)

From the trained weights w*, run a short tempered, anchored Stochastic Gradient Langevin Dynamics (SGLD) chain that samples the local posterior pinned to the basin by a Gaussian anchor (gamma/2)*||w - w*||^2, then form

LLC_hat = n * beta * ( mean_chain[L(w)] - L(w*) )

where L is the average training NLL over the probe set, n is the sample count and beta = 1/ln(n) is the WBIC inverse temperature. The per-step anchored-Langevin update over a minibatch loss gradient g is

w <- w - (eps/2)*( n*beta*g + gamma*(w - w*) ) + N(0, eps)

The report reuses the existing forward+backward gradient machinery (the only new piece is the anchored update + chain average). It is non-destructive: w* is snapshotted and restored bit-for-bit on return.

What this example shows

Three nets are fit (or not) to the same 3-class toy target:

1. minimal net, trained to fit;
2. over-parameterised net, trained to fit, then two hidden units are forced redundant (duplicate-then-halve) so two directions are exactly flat;
3. random-init net, never trained.

Robust, reproducible reading

• Both trained nets (1) & (2) report a small LLC_hat with LLC_hat << dim(w): the basin has far fewer effective degrees of freedom than raw weights.
• The random-init net (3) reports a large, often negative LLC_hat. The LLC is only defined at a local minimum; at a random point the anchored chain slides downhill so mean_chain[L] < L(w*). A negative / wild LLC_hat is the diagnostic's honest "this is not a minimum" signal.

Run it (pure CPU, ~3 seconds):

lazbuild LocalLearningCoefficient.lpi
../../bin/x86_64-linux/bin/LocalLearningCoefficient

Known pitfall — and what did NOT fit the CPU budget

LLC estimates are sensitive to eps, gamma and the chain length. This example ships fixed, documented values (chain=300, eps=1e-4, gamma=10); the absolute LLC_hat is calibration-dependent and shifts when you change them. Only the ordering of LLC_hat across nets measured with the same fixed hyperparameters is meaningful.

The fine-grained (1)-vs-(2) ordering (a minimal net below an over-parameterised one fitting the same function) is real in expectation, but at the short fixed chain length affordable on a CPU toy the run-to-run estimator noise on the tiny mean_chain[L] - L(w*) gap is the same size as that gap, so the two trained arms do not separate reproducibly across random seeds. The robust headline here is therefore the trained-vs-untrained contrast plus LLC_hat << dim(w). Resolving (1) vs (2) cleanly would need a much longer chain (and several chains averaged) — beyond this example's budget.

The report itself never steps the weights (a measurement, not training).