From db0c64bbe6292af1b99cb8575dca3f48828ba84b Mon Sep 17 00:00:00 2001
From: james <li@jamesdeMacBook-Pro.local>
Date: Mon, 18 May 2026 21:03:35 -0700
Subject: [PATCH 1/2] Add maths/numerical_analysis/lagrange_interpolation.py

Implements Lagrange polynomial interpolation for arbitrarily spaced
data points. Unlike Newton's forward-difference formula (which requires
equidistant x-values), Lagrange interpolation works with any distinct
x_points.

Includes full type hints, 9 doctests, and raises ValueError for
mismatched lengths, too few points, or duplicate x-values.

References:
- https://en.wikipedia.org/wiki/Lagrange_polynomial
- https://mathworld.wolfram.com/LagrangeInterpolatingPolynomial.html
---
 .../lagrange_interpolation.py                 | 94 +++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 maths/numerical_analysis/lagrange_interpolation.py

diff --git a/maths/numerical_analysis/lagrange_interpolation.py b/maths/numerical_analysis/lagrange_interpolation.py
new file mode 100644
index 000000000000..1cec52349fcb
--- /dev/null
+++ b/maths/numerical_analysis/lagrange_interpolation.py
@@ -0,0 +1,94 @@
+"""
+Lagrange Interpolation
+
+Given n+1 data points (x0, y0), (x1, y1), ..., (xn, yn) with distinct x-values,
+the Lagrange interpolating polynomial is the unique polynomial of degree <= n that
+passes through all given points.
+
+For each i, the basis polynomial is:
+    L_i(x) = product_{j != i} (x - x_j) / (x_i - x_j)
+
+The interpolating polynomial is:
+    P(x) = sum_i y_i * L_i(x)
+
+Unlike Newton's forward-difference formula, Lagrange interpolation works for
+arbitrarily spaced data points.
+
+References:
+    - https://en.wikipedia.org/wiki/Lagrange_polynomial
+    - https://mathworld.wolfram.com/LagrangeInterpolatingPolynomial.html
+"""
+
+from __future__ import annotations
+
+
+def lagrange_interpolation(x_points: list[float], y_points: list[float], x: float) -> float:
+    """
+    Estimate f(x) using the Lagrange interpolating polynomial through the
+    given data points (x_points[i], y_points[i]).
+
+    :param x_points: List of distinct x-coordinates of the data points.
+    :param y_points: List of y-coordinates corresponding to x_points.
+    :param x: The x-value at which to evaluate the interpolating polynomial.
+    :return: The interpolated value P(x).
+    :raises ValueError: If x_points and y_points have different lengths,
+                        if fewer than 2 points are given, or if x_points
+                        contains duplicate values.
+
+    Examples — interpolating through known points of f(x) = x^2:
+    >>> lagrange_interpolation([1, 2, 3], [1, 4, 9], 2.5)
+    6.25
+    >>> lagrange_interpolation([1, 2, 3], [1, 4, 9], 1.5)
+    2.25
+    >>> lagrange_interpolation([1, 2, 3], [1, 4, 9], 3.0)
+    9.0
+
+    Two-point linear interpolation:
+    >>> lagrange_interpolation([0, 1], [0, 1], 0.5)
+    0.5
+    >>> lagrange_interpolation([0, 2], [0, 4], 1.0)
+    2.0
+
+    Four points from f(x) = x^3, interpolated at a non-grid point:
+    >>> lagrange_interpolation([0, 1, 2, 3], [0, 1, 8, 27], 1.5)
+    3.375
+
+    Error cases:
+    >>> lagrange_interpolation([1, 2], [1], 1.5)
+    Traceback (most recent call last):
+        ...
+    ValueError: x_points and y_points must have the same length
+    >>> lagrange_interpolation([1], [1], 1.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: at least 2 data points are required
+    >>> lagrange_interpolation([1, 1, 3], [1, 2, 3], 2.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: x_points must be distinct (duplicate found: 1)
+    """
+    if len(x_points) != len(y_points):
+        raise ValueError("x_points and y_points must have the same length")
+    n = len(x_points)
+    if n < 2:
+        raise ValueError("at least 2 data points are required")
+    seen: set[float] = set()
+    for val in x_points:
+        if val in seen:
+            raise ValueError(f"x_points must be distinct (duplicate found: {val})")
+        seen.add(val)
+
+    result = 0.0
+    for i in range(n):
+        basis = 1.0
+        for j in range(n):
+            if j != i:
+                basis *= (x - x_points[j]) / (x_points[i] - x_points[j])
+        result += y_points[i] * basis
+    return result
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()

From 5a6260ce3111c331467151a7ea0acfe1c45b8b56 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 19 May 2026 04:04:00 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 maths/numerical_analysis/lagrange_interpolation.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/maths/numerical_analysis/lagrange_interpolation.py b/maths/numerical_analysis/lagrange_interpolation.py
index 1cec52349fcb..459678fb6805 100644
--- a/maths/numerical_analysis/lagrange_interpolation.py
+++ b/maths/numerical_analysis/lagrange_interpolation.py
@@ -22,7 +22,9 @@
 from __future__ import annotations
 
 
-def lagrange_interpolation(x_points: list[float], y_points: list[float], x: float) -> float:
+def lagrange_interpolation(
+    x_points: list[float], y_points: list[float], x: float
+) -> float:
     """
     Estimate f(x) using the Lagrange interpolating polynomial through the
     given data points (x_points[i], y_points[i]).