Nonlinear Solvers

Goal

Provide a universal workflow to select a nonlinear solver, configure globalization strategies, and diagnose convergence for root-finding, optimization, and least-squares problems.

Requirements

• Python 3.8+
• NumPy (for Jacobian diagnostics)
• SciPy (optional, for advanced analysis)

Inputs to Gather

Decision Guidance

Solver Selection Flowchart

Is Jacobian available and cheap?
├── YES → Problem size?
│   ├── Small (n < 1000) → Newton (full)
│   └── Large (n ≥ 1000) → Newton-Krylov
└── NO → Is objective smooth?
    ├── YES → Memory limited?
    │   ├── YES → L-BFGS or Broyden
    │   └── NO → BFGS
    └── NO → Anderson acceleration or Picard

Quick Reference

Script Outputs (JSON Fields)

Workflow

1. Characterize problem - Identify type, size, Jacobian availability
2. Select solver - Run scripts/solver_selector.py
3. Choose globalization - Run scripts/globalization_advisor.py
4. Analyze Jacobian - If available, run scripts/jacobian_diagnostics.py
5. Monitor residuals - During solve, use scripts/residual_monitor.py
6. Analyze convergence - Run scripts/convergence_analyzer.py
7. Evaluate steps - For trust region, use scripts/step_quality.py

Conversational Workflow Example

User: My Newton solver for a phase-field simulation is converging very slowly. After 50 iterations, the residual only dropped from 1 to 0.1.

Agent workflow:

1. Analyze convergence:
   bashCopy

   python3 scripts/convergence_analyzer.py --residuals 1,0.8,0.6,0.5,0.4,0.3,0.2,0.15,0.12,0.1 --json
   

2. Check globalization strategy:
   bashCopy

   python3 scripts/globalization_advisor.py --problem-type root-finding --jacobian-quality ill-conditioned --previous-failures 0 --json
   

3. Recommend: Switch to trust region with Levenberg-Marquardt regularization, or use Newton-Krylov with better preconditioning.

Pre-Solve Checklist

• Confirm problem type (root-finding, optimization, least-squares)
• Assess Jacobian availability and cost
• Check initial guess quality
• Set appropriate tolerances
• Choose globalization strategy
• Prepare to monitor convergence

CLI Examples

# Select solver for large unconstrained optimization
python3 scripts/solver_selector.py --size 50000 --smooth --memory-limited --json

# Analyze convergence from residual history
python3 scripts/convergence_analyzer.py --residuals 1,0.1,0.01,0.001,0.0001 --tolerance 1e-6 --json

# Diagnose Jacobian quality
python3 scripts/jacobian_diagnostics.py --matrix jacobian.txt --json

# Get globalization recommendation
python3 scripts/globalization_advisor.py --problem-type optimization --jacobian-quality good --json

# Monitor residual patterns
python3 scripts/residual_monitor.py --residuals 1,0.8,0.9,0.7,0.75,0.6 --target-tolerance 1e-8 --json

# Evaluate step quality for trust region
python3 scripts/step_quality.py --predicted-reduction 0.5 --actual-reduction 0.4 --step-norm 0.8 --gradient-norm 1.0 --trust-radius 1.0 --json

Error Handling

Interpretation Guidance

Convergence Type

Jacobian Quality

Step Quality (Trust Region)

Limitations

• No global convergence guarantee: All methods may fail for pathological problems
• Jacobian accuracy: Finite-difference Jacobian may be inaccurate near discontinuities
• Large dense problems: May require specialized solvers not covered here
• Constrained optimization: Complex constraints need SQP or interior point methods

References

• references/solver_decision_tree.md - Problem-based solver selection
• references/method_catalog.md - Method details and parameters
• references/convergence_diagnostics.md - Diagnosing convergence issues
• references/globalization_strategies.md - Line search and trust region

Version History

• v1.0.0 : Initial release with 6 analysis scripts