---
license: mit
tags:
- pytorch
- safetensors
- formal-verification
- coq
- mod3
- modular-arithmetic
- threshold-network
- neuromorphic
---

# tiny-mod3-prover

Formally verified neural network that computes the MOD-3 function (Hamming weight mod 3) on 8-bit inputs. This repository contains the model artifacts; for proof development and Coq source code, see [mod3-verified](https://github.com/CharlesCNorton/mod3-verified).

## Overview

This is a threshold network that computes `mod3(x) = HW(x) mod 3` for 8-bit binary inputs, where HW denotes Hamming weight (number of set bits). The network outputs 0, 1, or 2 corresponding to the three residue classes.

**Key properties:**
- 100% accuracy on all 256 possible inputs
- Correctness proven in Coq via constructive algebraic proof
- Weights constrained to integers (many ternary)
- Heaviside step activation (x ≥ 0 → 1, else 0)
- First formally verified threshold circuit for MOD-m where m > 2

## Architecture

| Layer | Neurons | Function |
|-------|---------|----------|
| Input | 8 | Binary input bits |
| Hidden 1 | 9 | Thermometer encoding (HW ≥ k) |
| Hidden 2 | 2 | MOD-3 detection |
| Output | 3 | Classification (one-hot) |

**Total: 14 neurons, 110 parameters**

## Quick Start

```python
import torch
from safetensors.torch import load_file

# Load weights
weights = load_file('model.safetensors')

# Manual forward pass (Heaviside activation)
def forward(x, weights):
    x = x.float()
    x = (x @ weights['layer1.weight'].T + weights['layer1.bias'] >= 0).float()
    x = (x @ weights['layer2.weight'].T + weights['layer2.bias'] >= 0).float()
    out = x @ weights['output.weight'].T + weights['output.bias']
    return out.argmax(dim=-1)

# Test
inputs = torch.tensor([[1, 0, 1, 1, 0, 0, 1, 0]], dtype=torch.float32)
output = forward(inputs, weights)
print(f"MOD-3 of [1,0,1,1,0,0,1,0]: {output.item()}")  # 1 (4 bits set, 4 mod 3 = 1)
```

## Weight Structure

| Tensor | Shape | Values | Description |
|--------|-------|--------|-------------|
| `layer1.weight` | [9, 8] | All 1s | Thermometer encoding |
| `layer1.bias` | [9] | [0, -1, ..., -8] | Threshold at HW ≥ k |
| `layer2.weight` | [2, 9] | [0,1,1,-2,1,1,-2,1,1] | MOD-3 detection |
| `layer2.bias` | [2] | [-1, -2] | Class thresholds |
| `output.weight` | [3, 2] | Various | Classification |
| `output.bias` | [3] | [0, -1, -1] | Output thresholds |

## Algebraic Insight

For parity (MOD-2), the key insight was that ±1 dot products preserve Hamming weight parity because (-1) ≡ 1 (mod 2).

For MOD-3, the insight is different. Using weights `(1, 1, -2)` repeated on the thermometer encoding produces partial sums that cycle through `(0, 1, 2, 0, 1, 2, ...)`:

```
HW=0: cumsum = 0  →  0 mod 3
HW=1: cumsum = 1  →  1 mod 3
HW=2: cumsum = 2  →  2 mod 3
HW=3: cumsum = 0  →  0 mod 3  (reset: 1+1-2=0)
HW=4: cumsum = 1  →  1 mod 3
HW=5: cumsum = 2  →  2 mod 3
HW=6: cumsum = 0  →  0 mod 3  (reset)
HW=7: cumsum = 1  →  1 mod 3
HW=8: cumsum = 2  →  2 mod 3
```

The pattern `1 + 1 + (-2) = 0` causes the cumulative sum to reset every 3 steps, tracking HW mod 3.

This generalizes to MOD-m: use weights `(1, 1, ..., 1, 1-m)` with `m-1` ones before the `1-m` term.

## Formal Verification

The network is proven correct in the Coq proof assistant with three independent proofs:

**1. Exhaustive verification:**
```coq
Theorem network_correct_exhaustive : verify_all = true.
Proof. vm_compute. reflexivity. Qed.
```

**2. Constructive verification (case analysis):**
```coq
Theorem network_correct_constructive : forall x0 x1 x2 x3 x4 x5 x6 x7,
  predict [x0; x1; x2; x3; x4; x5; x6; x7] =
  mod3 [x0; x1; x2; x3; x4; x5; x6; x7].
```

**3. Algebraic verification:**
```coq
Theorem cumsum_eq_mod3 : forall k,
  (k <= 8)%nat -> cumsum k = Z.of_nat (Nat.modulo k 3).

Theorem network_algebraic_correct : forall h,
  (h <= 8)%nat ->
  classify (Z.geb (cumsum h - 1) 0) (Z.geb (cumsum h - 2) 0) = Nat.modulo h 3.
```

All proofs are axiom-free ("Closed under the global context").

## MOD-3 Distribution

For 8-bit inputs (256 total):

| Class | Count | Hamming Weights |
|-------|-------|-----------------|
| 0 | 85 | 0, 3, 6 |
| 1 | 86 | 1, 4, 7 |
| 2 | 85 | 2, 5, 8 |

## Training

The parametric construction was derived algebraically, not discovered through training.

Evolutionary search was attempted (as with parity) but consistently plateaued at 247/256 (96.5%) accuracy across multiple seeds. We were unable to discover the (1,1,-2) weight pattern through training, so we used algebraic construction instead.

## Comparison to Parity

| Property | Parity (MOD-2) | MOD-3 |
|----------|----------------|-------|
| Output classes | 2 | 3 |
| Key weights | ±1 (any) | (1,1,-2) specific |
| Training | Evolutionary (10K gen) | Algebraic construction |
| Neurons | 14 (pruned) | 14 |
| Parameters | 139 (pruned) | 110 |
| Algebraic insight | (-1) ≡ 1 (mod 2) | 1+1-2 = 0 (reset) |

## Limitations

- **Fixed input size**: 8 bits only (algebraic construction extends to any n)
- **Binary inputs**: Expects {0, 1}, not continuous values
- **No noise margin**: Heaviside threshold at exactly 0
- **Not differentiable**: Cannot be fine-tuned with gradient descent
- **Training gap**: Our evolutionary search achieved only 96.5%; we used algebraic construction instead

## Files

```
mod3-verified/
├── model.safetensors    # Network weights (110 params)
├── model.py             # Inference code
├── config.json          # Model metadata
└── README.md            # This file
```

## Citation

```bibtex
@software{tiny_mod3_prover_2025,
  title={tiny-mod3-prover: Formally Verified Threshold Network for MOD-3},
  author={Norton, Charles},
  url={https://huggingface.co/phanerozoic/tiny-mod3-prover},
  year={2025},
  note={First verified threshold circuit for modular counting beyond parity}
}
```

## Related

- **Proof repository**: [mod3-verified](https://github.com/CharlesCNorton/mod3-verified) — Coq proofs, training attempts, full documentation
- **Parity network**: [tiny-parity-prover](https://huggingface.co/phanerozoic/tiny-parity-prover) — Verified MOD-2 (parity) network
- **Parity proofs**: [threshold-logic-verified](https://github.com/CharlesCNorton/threshold-logic-verified) — Original parity verification project

## License

MIT