Itô isometry

In

Let ${\displaystyle W:[0,T]\times \Omega \to \mathbb {R} }$ denote the canonical real-valued Wiener process defined up to time ${\displaystyle T>0}$, and let ${\displaystyle X:[0,T]\times \Omega \to \mathbb {R} }$ be a stochastic process that is adapted to the natural filtration ${\displaystyle {\mathcal {F}}_{*}^{W}}$ of the Wiener process.^{[clarification needed]} Then

${\displaystyle \operatorname {E} \left[\left(\int _{0}^{T}X_{t}\,\mathrm {d} W_{t}\right)^{2}\right]=\operatorname {E} \left[\int _{0}^{T}X_{t}^{2}\,\mathrm {d} t\right],}$

where ${\displaystyle \operatorname {E} }$ denotes

In other words, the Itô integral, as a function from the space ${\displaystyle L_{\mathrm {ad} }^{2}([0,T]\times \Omega )}$ of square-integrable adapted processes to the space ${\displaystyle L^{2}(\Omega )}$ of square-integrable random variables, is an

${\displaystyle {\begin{aligned}(X,Y)_{L_{\mathrm {ad} }^{2}([0,T]\times \Omega )}&:=\operatorname {E} \left(\int _{0}^{T}X_{t}\,Y_{t}\,\mathrm {d} t\right)\end{aligned}}}$

and

${\displaystyle (A,B)_{L^{2}(\Omega )}:=\operatorname {E} (AB).}$

As a consequence, the Itô integral respects these inner products as well, i.e. we can write

${\displaystyle \operatorname {E} \left[\left(\int _{0}^{T}X_{t}\,\mathrm {d} W_{t}\right)\left(\int _{0}^{T}Y_{t}\,\mathrm {d} W_{t}\right)\right]=\operatorname {E} \left[\int _{0}^{T}X_{t}Y_{t}\,\mathrm {d} t\right]}$

for ${\displaystyle X,Y\in L_{\mathrm {ad} }^{2}([0,T]\times \Omega )}$ .

The Itô isometry can be illustrated through numerical simulation using

and computing both sides of the isometry equation for different choices of the integrand process ${\displaystyle X_{t}}$. The simulation approximates the continuous-time

stochastic integral

using a discrete-time summation:

${\displaystyle \int _{0}^{T}X_{t}\,dW_{t}\approx \sum _{i=0}^{N-1}X_{t_{i}}\Delta W_{i}}$

where ${\displaystyle \Delta W_{i}=W_{t_{i+1}}-W_{t_{i}}}$ represents the Brownian increments over small time intervals. The isometry can be demonstrated using various processes ${\displaystyle X_{t}}$ on the interval ${\displaystyle [0,1]}$:

1. Constant process: ${\displaystyle X_{t}=1}$
   • Analytical value: ${\displaystyle \mathbb {E} [\int _{0}^{1}1^{2}\,dt]=1}$,
2. Linear deterministic process: ${\displaystyle X_{t}=t}$
   • Analytical value: ${\displaystyle \mathbb {E} [\int _{0}^{1}t^{2}\,dt]={\frac {1}{3}}}$,
3. Trigonometric deterministic process: ${\displaystyle X_{t}=\sin(\pi t)}$
   • Analytical value: ${\displaystyle \mathbb {E} [\int _{0}^{1}\sin ^{2}(\pi t)\,dt]={\frac {1}{2}}}$,
4. Path-dependent stochastic process: ${\displaystyle X_{t}=W_{t}}$
   • Analytical value: ${\displaystyle \mathbb {E} [\int _{0}^{1}W_{t}^{2}\,dt]={\frac {1}{2}}}$,
5. Compensated Poisson (${\displaystyle M_{t}=N_{t}-\lambda t}$): ${\displaystyle X_{t}=1}$
   • Analytical value: ${\displaystyle \lambda \int _{0}^{T}1^{2}dt=\lambda T}$.

# ============================================================
#  Convergence test for five processes
#  – 4 Brownian‑based Itô integrals
#  – 1 compensated‑Poisson martingale integral
#
#  Output:
#    • nicely formatted error table
#    • log–log convergence plot
#      – each Brownian curve in its own colour
#      – Poisson curve orange & dashed
# ============================================================

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from IPython.display import display

np.random.seed(42)  # reproducible demo

# ---------------------- global settings ---------------------
T = 1.0  # horizon
lam = 3.0  # Poisson intensity λ
M_paths = 20_000  # Monte‑Carlo paths
N_list = [50, 100, 200, 500, 1000, 2000, 5000]  # mesh refinements

# dataframe to hold absolute errors
index = pd.Index(N_list, name="N")
err_table = pd.DataFrame(index=index, columns=["1", "t", "sin(πt)", "W_t", "Poisson‑1"])

# -------------------- main simulation loop ------------------
for N in N_list:
    dt = T / N
    sqrt_dt = np.sqrt(dt)
    t_left = np.linspace(0.0, T, N + 1)[:-1]  # left endpoints (length N)

    # --- Brownian increments & paths ------------------------
    dW = np.random.normal(0.0, sqrt_dt, size=(M_paths, N))
    W = np.zeros((M_paths, N + 1))
    W[:, 1:] = np.cumsum(dW, axis=1)

    # --- Compensated Poisson increments --------------------
    dN = np.random.poisson(lam * dt, size=(M_paths, N))
    dM = dN - lam * dt

    # helper for deterministic X_t
    lhs_det = lambda X_grid, inc: np.mean((inc * X_grid).sum(axis=1) ** 2)

    # 1) X_t ≡ 1  with Brownian integrator
    X1 = np.ones_like(t_left)
    err_table.loc[N, "1"] = abs(lhs_det(X1, dW) - T)

    # 2) X_t = t
    Xt = t_left
    err_table.loc[N, "t"] = abs(lhs_det(Xt, dW) - T**3 / 3)

    # 3) X_t = sin(π t)
    Xs = np.sin(np.pi * t_left)
    err_table.loc[N, "sin(πt)"] = abs(lhs_det(Xs, dW) - 0.5 * T)

    # 4) X_t = W_t   (path‑dependent)
    lhs_W = np.mean((W[:, :-1] * dW).sum(axis=1) ** 2)
    err_table.loc[N, "W_t"] = abs(lhs_W - T**2 / 2)

    # 5) compensated Poisson with X_t ≡ 1
    err_table.loc[N, "Poisson‑1"] = abs(lhs_det(X1, dM) - lam * T)

# ---------------------- show table --------------------------
display(
    err_table.style.format("{:.3e}").set_caption(
        "Absolute error of isometry vs N (5 processes)"
    )
)

# ---------------------- plotting ----------------------------
colour_map = {
    "1": "tab:blue",
    "t": "tab:green",
    "sin(πt)": "tab:red",
    "W_t": "tab:purple",
    "Poisson‑1": "tab:orange",
}
markers = {"1": "o", "t": "s", "sin(πt)": "D", "W_t": "^", "Poisson‑1": "v"}
styles = {"1": "-", "t": "-", "sin(πt)": "-", "W_t": "-", "Poisson‑1": "--"}

plt.figure(figsize=(7, 5))
for col in err_table.columns:
    plt.plot(
        N_list,
        err_table[col],
        marker=markers[col],
        linestyle=styles[col],
        color=colour_map[col],
        label=col,
    )

plt.xscale("log")
plt.yscale("log")
plt.xlabel("time‑step count $N$  (log scale)")
plt.ylabel("absolute error  (log scale)")
plt.title(
    "Convergence of isometry error vs $N$\n"
    "(4 Brownian processes, 1 compensated‑Poisson process)"
)
plt.grid(True, which="both", ls=":")
plt.legend()
plt.tight_layout()
plt.show()

A Monte Carlo simulation with 20,000 sample paths and 1,000 time steps produces results that closely match the theoretical values predicted by the Itô isometry. As shown in the table above, the absolute errors between the simulated left-hand side ${\displaystyle \mathbb {E} [(\int X_{t}\,dW_{t})^{2}]}$ and the analytical right-hand side ${\displaystyle \mathbb {E} [\int X_{t}^{2}\,dt]}$ are typically on the order of ${\displaystyle 10^{-3}}$ or smaller, confirming the validity of the isometry relationship.

These simulations serve as empirical evidence for the Itô isometry and provide insight into how the relationship holds across different types of processes, both deterministic and stochastic. The close agreement between theoretical and simulated values demonstrates the robustness of the isometry as a fundamental property of stochastic calculus.

The Itô isometry extends beyond the standard

${\displaystyle \{X_{t}\}_{t\geq 0}}$ is a

${\displaystyle \{{\mathcal {F}}_{t}\}_{t\geq 0}}$

• ${\displaystyle \mathbb {E} [X_{t}|{\mathcal {F}}_{s}]=X_{s}}$ for all ${\displaystyle t\geq s}$

Martingales can be further classified as:

• ${\displaystyle L^{1}}$ martingales if ${\displaystyle \mathbb {E} [|X_{t}|]<\infty }$ for all ${\displaystyle t}$
• ${\displaystyle L^{2}}$ martingales if ${\displaystyle \mathbb {E} [|X_{t}|^{2}]<\infty }$for all ${\displaystyle t}$ (and by implication, also ${\displaystyle L^{1}}$)

A local martingale is a process ${\displaystyle \{X_{t}\}}$ for which there exists a sequence of stopping times ${\displaystyle \{\tau _{n}\}}$ with ${\displaystyle \tau _{n}\rightarrow \infty }$ such that ${\displaystyle \{X_{t\wedge \tau _{n}}\}}$ is a martingale for each ${\displaystyle n}$.

The

. For a predictable process ${\displaystyle X_{t}}$ and an appropriate integrator ${\displaystyle Y_{t}}$, the

Itô integral

is defined as the limit in ${\displaystyle L^{2}}$ of simple predictable processes approximating ${\displaystyle X_{t}}$:

${\displaystyle \int _{0}^{t}X_{s}\,dY_{s}=\lim _{n\to \infty }\sum _{i=1}^{n-1}\xi _{i}(Y_{t_{i+1}}-Y_{t_{i}})}$

where each ${\displaystyle \xi _{i}}$ is ${\displaystyle {\mathcal {F}}_{t_{i}}}$-measurable.

The Itô isometry holds when the integrator ${\displaystyle Y_{t}}$ is one of:

• An ${\displaystyle L^{1}}$ martingale
• An ${\displaystyle L^{2}}$ martingale
• A local ${\displaystyle L^{1}}$ or ${\displaystyle L^{2}}$ martingale

For these cases, the isometry takes the form:

${\displaystyle \mathbb {E} \left[\left(\int _{0}^{T}X_{s}\,dY_{s}\right)^{2}\right]=\mathbb {E} \left[\int _{0}^{T}X_{s}^{2}\,d[Y]_{s}\right]}$

where ${\displaystyle [Y]_{s}}$ denotes the quadratic variation process of ${\displaystyle Y_{s}}$.

• ISBN 3-540-04758-1
  .