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Learning from Integral Losses in Physics Informed Neural Networks
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Shows that replacing integrals in PINN losses with unbiased single-sample estimates induces a parameter-dependent variance bias, and proposes deterministic sampling, double sampling, and a regularized delayed target method (N=1) that matches large-N integral estimates on Poisson, Maxwell-Ampere, and Smoluchowski problems.
Problem
Training physics-informed neural networks on partial integro-differential equations, where integrals or large sums in the PDE residual make accurate per-point evaluation impractical and naive unbiased sampling yields biased losses.
Approach
Shows the naive sampled MSE loss equals the true loss plus an excess variance term that depends on theta, then reduces that bias by using a detached, Polyak-averaged target network for the integral term (delayed target), with an added regularizer tying main and target outputs.
Key results
- Delayed target with N=1 achieves solution quality comparable to or slightly better than standard training with N=100 on Poisson, Maxwell, and Smoluchowski problems.
- Naive N=1 standard training yields overly smooth, biased solutions due to excess variance; double-sampling with N=1 diverges on the Poisson problem.
- Regularized delayed target (lambda=1) stabilizes training with M=1000 where unregularized M=100 diverges.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| use delayed targets with N=1 rather than standard training with N=1 | PINNs on the Poisson problem with singular charges · Poisson problem with singular charges | solution quality | Helped |
| use delayed targets with N=1 rather than standard training with N=100 | PINNs on the Poisson problem with singular charges · Poisson problem with singular charges | solution quality | No clear effect |
| increase standard integral samples from N=1 to N=100 (ablation) | Standard training on Poisson with singular charges · Poisson problem with singular charges | solution quality | Helped |
| increase delayed-target M from 10 to 100 (ablation) | Delayed target on Poisson with singular charges · Poisson problem with singular charges | training stability | Hurt |
| add target regularization and use M=1000 (ablation) | Delayed target on Poisson with singular charges · Poisson problem with singular charges | training stability | Helped |
| replace i.i.d. sampling with deterministic sampling at N=1 (ablation) | PINNs on the Poisson problem with singular charges · Poisson problem with singular charges | solution quality | Hurt |
| use double sampling at N=1 (ablation) | PINNs on the Poisson problem with singular charges · Poisson problem with singular charges | training stability | Hurt |
| increase deterministic and double-sampling integral estimation from N=1 to N=100 (ablation) | PINNs on the Poisson problem with singular charges · Poisson problem with singular charges | solution quality | Helped |
| use delayed targets with small N instead of standard training with large N | PINNs on the Maxwell-Ampere problem · Maxwell-Ampere problem with a wired circuit | solution quality | No clear effect |
| use double sampling with small N (ablation) | PINNs on the Maxwell-Ampere problem · Maxwell-Ampere problem with a wired circuit | solution quality | Mixed |
| use standard training with N=1 rather than N=100 (ablation) | PINNs on the Smoluchowski coagulation problem · Smoluchowski coagulation problem | solution quality | Hurt |
| use delayed targets with N=1 rather than standard training with N=100 | PINNs on the Smoluchowski coagulation problem · Smoluchowski coagulation problem | solution quality | Helped |
Surprising findings
- The double-sampling trick, which gives an unbiased full gradient, performed worse than the biased delayed target method in practice.
- Incomplete-gradient (TD-style) updates were preferable to full-gradient updates, consistent with off-policy RL findings.
Limitations
- Convergence guarantee holds only under linear function approximation and a discretized domain; nonlinear networks may not converge to reasonable solutions.
- Hyperparameters such as M, lambda, and batch size require manual setting; adaptive schemes left to future work.
- Experiments are on relatively small MLPs and synthetic PDE problems.
Experimental setup
Datasets: synthetic Poisson problems with Dirac-delta charges Maxwell-Ampere circuit problem Smoluchowski coagulation problem
Benchmarks: 2D Poisson with singular charges higher-dimensional Poisson (up to 10D) Maxwell-Ampere wired circuit Smoluchowski coagulation (2-4D)
Metrics: mean squared error to analytical solution normalized MSE
Baselines: standard MSE training with N=1 standard MSE training with N=100 deterministic sampling double-sampling trick Gaussian quadrature Leja quadrature quasi Monte Carlo
Builds on: physics-informed neural networks temporal difference learning double sampling Adam
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