optimiz-rs/docs/source/algorithms/point_processes.md

# Point Processes & Fractional Brownian Motion

This module implements the mathematical framework from **Muhle-Karbe, Jusselin & Rosenbaum** (2022) for modeling order flow microstructure through self-exciting point processes and fractional dynamics.

It provides high-performance Rust implementations of:
- **Hawkes Processes** with flexible excitation kernels
- **Fractional Brownian Motion (fBM)** with exact simulation
- **Mixed Fractional Brownian Motion (mfBM)** for aggregate flow
- **Mittag-Leffler Functions** for scaling limit analysis

---

## Mathematical Foundations

### The Unified Theory of Order Flow

The key insight from the unified theory is that a **single parameter** — the Hurst exponent $H_0 \approx 3/4$ — governs all market microstructure quantities:

$$
\boxed{H_0 \approx \frac{3}{4}}
$$

This parameter determines:

| Quantity | Formula | Value at $H_0 = 3/4$ |
|----------|---------|----------------------|
| Price roughness | $H_{\text{price}} = H_0 - \tfrac{1}{2}$ | $1/4$ |
| Volatility roughness | $H_{\text{vol}} \approx H_0 - \tfrac{1}{2}$ | $\approx 0.1$ |
| Market impact exponent | $\delta = 1 - \tfrac{1}{2H_0}$ | $1/3$ |
| Kyle's lambda | $\Lambda \sim n^{-\delta}$ | $\sim n^{-1/3}$ |
| Kernel tail exponent | $\alpha_0 = H_0/2$ | $3/8$ |

The model structure is:

$$
N = F + R
$$

where:
- $N$ = total order flow (observable)
- $F$ = core (fundamental) order flow
- $R$ = reaction (self-exciting) order flow modeled by Hawkes processes

---

## Hawkes Processes

### Definition

A (univariate) Hawkes process $N(t)$ has conditional intensity:

$$
\lambda(t) = \nu + \int_0^{t^-} \phi(t - s) \, dN(s) = \nu + \sum_{t_i < t} \phi(t - t_i)
$$

where:
- $\nu > 0$ is the **baseline intensity** (exogenous arrival rate)
- $\phi: \mathbb{R}_+ \to \mathbb{R}_+$ is the **excitation kernel** (self-exciting memory)
- $t_i$ are past event times

The process is **stable** (stationary) when the **branching ratio** satisfies:

$$
\|\phi\|_{L^1} = \int_0^\infty \phi(t) \, dt < 1
$$

The expected number of events per unit time in stationarity is:

$$
\mathbb{E}[\lambda] = \frac{\nu}{1 - \|\phi\|_{L^1}}
$$

### Excitation Kernels

#### Exponential Kernel (Short Memory)

$$
\phi(t) = \alpha \, e^{-\beta t}, \quad \alpha, \beta > 0
$$

Properties:
- **L¹ norm**: $\|\phi\|_{L^1} = \alpha / \beta$
- **Stability**: $\alpha < \beta$
- **Half-life**: $t_{1/2} = \ln 2 / \beta$
- **Tail**: exponential decay (no long memory)
- **Characteristic timescale**: $\tau = 1/\beta$

The exponential kernel leads to an intensity process that is Markovian — the full history can be summarized by the current intensity level. The integrated kernel is:

$$
\int_0^t \phi(s) \, ds = \frac{\alpha}{\beta} \left(1 - e^{-\beta t}\right)
$$

#### Power-Law Kernel (Long Memory)

$$
\phi(t) = K_0 \, (1 + t)^{-(1 + \alpha_0)}, \quad K_0 > 0, \; \alpha_0 \in (0, 1)
$$

Properties:
- **L¹ norm**: $\|\phi\|_{L^1} = K_0 / \alpha_0$
- **Stability**: $K_0 < \alpha_0$
- **Tail exponent**: $\alpha_0$ controls memory persistence
- **Hurst connection**: $H_0 = 2\alpha_0$ (from the unified theory)
- **Long memory**: polynomial decay produces clustering at all timescales

The integrated kernel is:

$$
\int_0^t \phi(s) \, ds = \frac{K_0}{\alpha_0} \left[1 - (1 + t)^{-\alpha_0}\right]
$$

The **critical** regime ($\|\phi\|_{L^1} = 1$) corresponds to $K_0 = \alpha_0$, and the **nearly-critical** regime ($\|\phi\|_{L^1} = 1 - \varepsilon$) is relevant for real market data where the branching ratio is very close to 1.

#### Completely Monotone Kernel (Assumption A)

From the unified theory paper's **Assumption A**, the most general kernel satisfying the scaling limit theorems:

$$
\phi(t) = K_0 \, t^{-\alpha_0} \, E_{1-\alpha_0}\!\left(-\lambda \, t^{1-\alpha_0}\right)
$$

where $E_\alpha$ is the Mittag-Leffler function. This kernel:
- Is **completely monotone** on $(0, \infty)$
- Interpolates between power-law and exponential behavior
- Satisfies all conditions for the scaling limit theorems

### Simulation: Ogata's Thinning Algorithm

The Hawkes process is simulated using **Ogata's thinning algorithm**:

1. Compute upper bound $\lambda_{\max} \geq \lambda(t)$ for the current intensity
2. Generate candidate inter-arrival time $\tau \sim \text{Exp}(\lambda_{\max})$
3. Accept with probability $\lambda(t + \tau) / \lambda_{\max}$
4. If rejected, advance time to $t + \tau$ and repeat

The algorithm has expected time complexity $O(n \log n)$ where $n$ is the number of events.

### Maximum Likelihood Estimation

The log-likelihood of a Hawkes process on $[0, T]$ with event times $\{t_1, \ldots, t_n\}$:

$$
\ell(\boldsymbol{\theta}) = \sum_{i=1}^n \log \lambda(t_i) - \int_0^T \lambda(t) \, dt
$$

The compensator (integrated intensity) decomposes as:

$$
\int_0^T \lambda(t) \, dt = \nu T + \sum_{i=1}^n \int_0^{T - t_i} \phi(s) \, ds
$$

### Bivariate Hawkes Process

For order flow modeling, buy and sell reaction orders follow a **bivariate Hawkes process** $\mathbf{N} = (N^+, N^-)$ with intensity:

$$
\begin{aligned}
\lambda^+(t) &= \mu^+(t) + \int \left[\phi_1(t-s) \, dN^+(s) + \phi_2(t-s) \, dN^-(s)\right] \\
\lambda^-(t) &= \mu^-(t) + \int \left[\phi_2(t-s) \, dN^+(s) + \phi_1(t-s) \, dN^-(s)\right]
\end{aligned}
$$

where:
- $\phi_1$: **self-excitation** kernel (buy $\to$ buy, sell $\to$ sell)
- $\phi_2$: **cross-excitation** kernel (buy $\to$ sell, sell $\to$ buy)
- $\mu^\pm(t)$: baselines driven by core order flow

The **stability condition** requires the spectral radius of the kernel matrix:

$$
\rho\!\left(\begin{pmatrix} \|\phi_1\|_1 & \|\phi_2\|_1 \\ \|\phi_2\|_1 & \|\phi_1\|_1 \end{pmatrix}\right) = \|\phi_1\|_1 + \|\phi_2\|_1 < 1
$$

The **signed flow** $N^+(t) - N^-(t)$ captures the net order imbalance driving price changes, while the **unsigned volume** $N^+(t) + N^-(t)$ measures total reaction activity.

---

## Fractional Brownian Motion

### Definition

Fractional Brownian motion (fBM) $B^H_t$ with **Hurst parameter** $H \in (0, 1)$ is the unique centered Gaussian process with:

$$
\text{Cov}(B^H_s, B^H_t) = \frac{1}{2}\left(|t|^{2H} + |s|^{2H} - |t-s|^{2H}\right)
$$

Key properties:
- **Self-similarity**: $B^H_{ct} \overset{d}{=} c^H B^H_t$ for all $c > 0$
- **Stationary increments**: $B^H_t - B^H_s \overset{d}{=} B^H_{t-s}$
- **Variance**: $\text{Var}(B^H_t) = t^{2H}$

The three regimes are:

| Range | Behavior | Autocorrelation | Financial Interpretation |
|-------|----------|----------------|------------------------|
| $H < 1/2$ | **Anti-persistent** (mean-reverting) | Negative | Price reversals dominate |
| $H = 1/2$ | **Standard BM** (no memory) | Zero | Random walk |
| $H > 1/2$ | **Persistent** (trending) | Positive | Trends persist |

### Fractional Gaussian Noise (fGn)

The increments of fBM form **fractional Gaussian noise** with autocovariance:

$$
\gamma(k) = \frac{1}{2}\left(|k-1|^{2H} - 2|k|^{2H} + |k+1|^{2H}\right)
$$

For $H > 1/2$, $\gamma(k) > 0$ for all $k$, indicating **long-range dependence**:

$$
\sum_{k=0}^\infty \gamma(k) = \infty
$$

### Simulation Methods

#### Cholesky Method

Exact simulation by forming the covariance matrix $\Sigma$ and computing its Cholesky decomposition:

$$
\Sigma = L L^\top, \quad \mathbf{B}^H = L \mathbf{Z}, \quad \mathbf{Z} \sim \mathcal{N}(\mathbf{0}, I_n)
$$

Complexity: $O(n^3)$ for decomposition, $O(n^2)$ for simulation.

#### Hosking's Method (Durbin-Levinson)

For regular time grids, uses the **Durbin-Levinson algorithm** to compute prediction coefficients for the fGn, then reconstructs fBM by cumulative summation:

1. Compute autocovariance sequence $\gamma(0), \gamma(1), \ldots, \gamma(n-1)$
2. Recursively compute Levinson coefficients $\phi_{i,j}$ and prediction variances $v_i$
3. Generate fGn: $X_i = \sum_{j=0}^{i-1} \phi_{i,j} X_{i-1-j} + \sqrt{v_i} Z_i$
4. Cumulate: $B^H_k = \sum_{i=0}^{k-1} X_i \cdot (\Delta t)^H$

Complexity: $O(n^2)$ — more efficient than Cholesky for large $n$.

### Hurst Exponent Estimation

#### Rescaled Range (R/S) Analysis

The R/S statistic for a subseries of length $n$:

$$
(R/S)_n = \frac{\max_{1 \leq k \leq n} W_k - \min_{1 \leq k \leq n} W_k}{S_n}
$$

where $W_k = \sum_{i=1}^k (X_i - \bar{X})$ is the cumulative deviation and $S_n$ is the standard deviation.

For fBM/fGn:

$$
\mathbb{E}[(R/S)_n] \sim c \cdot n^H \quad \text{as } n \to \infty
$$

The Hurst exponent is estimated by linear regression of $\log(R/S)$ against $\log n$:

$$
\hat{H} = \frac{\sum_i (\log n_i - \overline{\log n})(\log(R/S)_i - \overline{\log(R/S)})}{\sum_i (\log n_i - \overline{\log n})^2}
$$

---

## Mixed Fractional Brownian Motion

### Definition

The mixed fBM (mfBM) combines a standard BM with an independent fBM:

$$
M^H(t) = a \cdot B(t) + b \cdot B^H(t)
$$

where:
- $B(t)$: standard Brownian motion (diffusive component)
- $B^H(t)$: fractional BM with Hurst index $H$
- $a, b$: mixing coefficients

### Covariance Structure

$$
\text{Cov}(M^H_s, M^H_t) = a^2 \min(s,t) + \frac{b^2}{2}\left(|t|^{2H} + |s|^{2H} - |t-s|^{2H}\right)
$$

### Role in the Unified Theory

In the scaling limit of the Hawkes-based order flow model, the aggregate order flow converges to:

$$
\frac{1}{\sqrt{n}} \sum_{i=1}^{\lfloor nt \rfloor} (N^+_i - N^-_i) \xrightarrow{d} \sigma_F \cdot M^{H_0}(t)
$$

where the Hurst exponent $H_0 = 2\alpha_0$ is determined by the kernel tail.

### Semimartingale Property

The mfBM is a **semimartingale** if and only if $H > 3/4$. This has pricing implications:
- For $H > 3/4$: classical stochastic calculus applies, no arbitrage
- For $H \leq 3/4$: not a semimartingale, requires fractional calculus

### Scale-Dependent Hurst Analysis

To identify a mfBM (vs pure fBM or BM), examine the **scale-dependent Hurst exponent**:

$$
H(\Delta) = \frac{1}{2} \cdot \frac{\log \text{Var}[X(t+2\Delta) - X(t)]}{\log \text{Var}[X(t+\Delta) - X(t)]} \cdot \frac{1}{\log 2}
$$

For pure fBM, $H(\Delta) \approx H$ at all scales. For mfBM:
- **Short timescales**: $H(\Delta) \to 1/2$ (BM dominates)
- **Long timescales**: $H(\Delta) \to H$ (fBM dominates)

This crossover behavior is a hallmark of the mixed process and matches empirical observations in order flow data.

---

## Mittag-Leffler Functions

### Definition

The generalized Mittag-Leffler function:

$$
E_{\alpha,\beta}(z) = \sum_{k=0}^\infty \frac{z^k}{\Gamma(\alpha k + \beta)}, \quad \alpha > 0, \; \beta > 0
$$

Special cases:
- $E_{1,1}(z) = e^z$ (exponential function)
- $E_{2,1}(z^2) = \cosh(z)$ (hyperbolic cosine)
- $E_{1,2}(z) = (e^z - 1)/z$ (exponential integral)

### Asymptotic Behavior

For $0 < \alpha < 1$ and large $|z|$:

$$
E_{\alpha,\beta}(z) \sim \begin{cases}
\frac{1}{\alpha} z^{(1-\beta)/\alpha} \exp\!\left(z^{1/\alpha}\right) & z \to +\infty \\[6pt]
-\sum_{k=1}^{p} \frac{z^{-k}}{\Gamma(\beta - \alpha k)} + O(|z|^{-p-1}) & z \to -\infty
\end{cases}
$$

### The $f_{\alpha_0, \lambda_0}$ Function

From **Theorem 3.1** of Muhle-Karbe et al., the key scaling function:

$$
f_{\alpha_0, \lambda_0}(x) = \lambda_0 \, x^{\alpha_0 - 1} \, E_{\alpha_0, \alpha_0}\!\left(-\lambda_0 \, x^{\alpha_0}\right)
$$

This function controls how the Hawkes process's self-excitation structure manifests in the scaling limit. Its integral satisfies:

$$
\int_0^t f_{\alpha_0, \lambda_0}(s) \, ds = t^{\alpha_0} \, E_{\alpha_0, \alpha_0 + 1}\!\left(-\lambda_0 \, t^{\alpha_0}\right)
$$

The function $f_{\alpha_0, \lambda_0}$ interpolates between:
- **Short times**: $f(x) \sim \lambda_0 x^{\alpha_0 - 1}$ (power-law singularity)
- **Long times**: $f(x) \sim x^{-1-\alpha_0}$ (power-law decay like the kernel)

---

## Usage Examples

### Simulating a Hawkes Process

```python
import optimizr
import numpy as np
import matplotlib.pyplot as plt

# Simulate with exponential kernel
events_exp = optimizr.simulate_hawkes(
    baseline=1.0,    # ν = 1.0
    alpha=0.5,       # α = 0.5
    beta=1.0,        # β = 1.0
    t_max=100.0,
    kernel_type="exponential",
    seed=42
)

# Simulate with power-law kernel (H₀ ≈ 0.75)
events_pl = optimizr.simulate_hawkes(
    baseline=0.1,
    alpha=0.35,      # K₀ = 0.35
    beta=0.375,      # α₀ = 0.375 → H₀ = 2 × 0.375 = 0.75
    t_max=100.0,
    kernel_type="power_law",
    seed=42
)

print(f"Exponential kernel: {len(events_exp)} events")
print(f"Power-law kernel: {len(events_pl)} events")
```

### Bivariate Buy/Sell Reaction Flow

```python
import optimizr
import numpy as np

# Generate core order flow (Poisson driver)
rng = np.random.default_rng(42)
core_buys = np.sort(rng.uniform(0, 100, 200))
core_sells = np.sort(rng.uniform(0, 100, 180))

# Simulate bivariate Hawkes reaction flow
buy_times, sell_times = optimizr.simulate_bivariate_hawkes(
    core_buy_times=core_buys,
    core_sell_times=core_sells,
    phi1_alpha=0.3,   # Self-excitation (buy→buy, sell→sell)
    phi1_beta=1.0,
    phi2_alpha=0.2,   # Cross-excitation (buy→sell, sell→buy)
    phi2_beta=1.0,
    t_max=100.0,
    seed=42
)

print(f"Reaction buys: {len(buy_times)}, Reaction sells: {len(sell_times)}")
print(f"Net order imbalance: {len(buy_times) - len(sell_times)}")

# Check stability
l1_phi1 = 0.3 / 1.0  # L¹ norm of self-excitation
l1_phi2 = 0.2 / 1.0  # L¹ norm of cross-excitation
spectral_radius = l1_phi1 + l1_phi2
print(f"Spectral radius: {spectral_radius:.2f} ({'stable' if spectral_radius < 1 else 'UNSTABLE'})")
```

### Simulating Fractional Brownian Motion

```python
import optimizr
import numpy as np
import matplotlib.pyplot as plt

# Simulate fBM paths with different Hurst exponents
fig, axes = plt.subplots(1, 3, figsize=(15, 4))

for i, h in enumerate([0.3, 0.5, 0.8]):
    path = optimizr.simulate_fbm(hurst=h, n=1000, dt=0.01, seed=42)
    
    # Estimate Hurst exponent from the path
    h_est = optimizr.estimate_hurst(path)
    
    axes[i].plot(path, linewidth=0.5)
    axes[i].set_title(f"H = {h:.1f} (estimated: {h_est:.3f})")
    axes[i].set_xlabel("Time step")

plt.suptitle("Fractional Brownian Motion Paths")
plt.tight_layout()
plt.show()
```
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### Mixed fBM for Aggregate Order Flow

```python
import optimizr
import numpy as np

# Simulate mixed fBM (BM + fBM with H₀ = 0.75)
path = optimizr.simulate_mixed_fbm(
    a=1.0,       # BM coefficient
    b=1.0,       # fBM coefficient
    hurst=0.75,  # H₀ from unified theory
    n=5000,
    dt=0.01,
    seed=42
)

# Scale-dependent Hurst analysis (identifies mfBM vs pure fBM)
scales = [10, 50, 100, 500, 1000, 2000]
hurst_by_scale = optimizr.scale_dependent_hurst(
    data=path,
    scales=scales
)

print("Scale-Dependent Hurst Exponents:")
print("-" * 35)
for scale, h in sorted(hurst_by_scale.items()):
    print(f"  Scale {scale:>5d}: H = {h:.4f}")
```

### Mittag-Leffler and Scaling Functions

```python
import optimizr
import numpy as np
import matplotlib.pyplot as plt

# Verify E_{1,1}(z) = exp(z)
z = 2.0
ml_value = optimizr.mittag_leffler_py(
    alpha=1.0, beta=1.0, z=z
)
print(f"E_{{1,1}}({z}) = {ml_value:.6f}")
print(f"exp({z})     = {np.exp(z):.6f}")

# Plot the scaling function f_{α₀,λ₀}(x)
x = np.linspace(0.01, 10, 500)
alpha_0 = 0.375  # From H₀ = 0.75
lambda_0 = 1.0

f_values = [optimizr.f_alpha_lambda_py(alpha_0, lambda_0, xi) for xi in x]

plt.figure(figsize=(10, 5))
plt.subplot(1, 2, 1)
plt.plot(x, f_values)
plt.xlabel('x')
plt.ylabel(r'$f_{\alpha_0, \lambda_0}(x)$')
plt.title(f'Scaling Function (α₀={alpha_0}, λ₀={lambda_0})')

plt.subplot(1, 2, 2)
plt.loglog(x, np.abs(f_values))
plt.xlabel('x (log)')
plt.ylabel(r'$|f_{\alpha_0, \lambda_0}(x)|$ (log)')
plt.title('Power-law decay in scaling limit')
plt.tight_layout()
plt.show()
```
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---

## Theoretical References

1. **Muhle-Karbe, Jusselin & Rosenbaum** (2022). *A unified approach to the analysis of high-frequency financial markets and limit order books.* Annals of Applied Probability.

2. **Jaisson & Rosenbaum** (2015). *Limit theorems for nearly unstable Hawkes processes.* Annals of Applied Probability, 25(2), 600-631.

3. **Bacry, Mastromatteo & Muzy** (2015). *Hawkes processes in finance.* Market Microstructure and Liquidity, 1(01), 1550005.

4. **Mandelbrot & Van Ness** (1968). *Fractional Brownian motions, fractional noises and applications.* SIAM Review, 10(4), 422-437.

5. **Gatheral, Jaisson & Rosenbaum** (2018). *Volatility is rough.* Quantitative Finance, 18(6), 933-949.

6. **Ogata** (1981). *On Lewis' simulation method for point processes.* IEEE Transactions on Information Theory, 27(1), 23-31.

7. **Hosking** (1984). *Modeling persistence in hydrological time series using fractional differencing.* Water Resources Research, 20(12), 1898-1908.