init

2026-02-12 23:22:11 +08:00
parent 7b09eb3d89
commit 89660bba4e
5988 changed files with 2517516 additions and 0 deletions
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+from numpy import arange, newaxis, hstack, prod, array
+from scipy import linalg
+
+
+def _central_diff_weights(Np, ndiv=1):
+    """
+    Return weights for an Np-point central derivative.
+
+    Assumes equally-spaced function points.
+
+    If weights are in the vector w, then
+    derivative is w[0] * f(x-ho*dx) + ... + w[-1] * f(x+h0*dx)
+
+    Parameters
+    ----------
+    Np : int
+        Number of points for the central derivative.
+    ndiv : int, optional
+        Number of divisions. Default is 1.
+
+    Returns
+    -------
+    w : ndarray
+        Weights for an Np-point central derivative. Its size is `Np`.
+
+    Notes
+    -----
+    Can be inaccurate for a large number of points.
+
+    Examples
+    --------
+    We can calculate a derivative value of a function.
+
+    >>> def f(x):
+    ...     return 2 * x**2 + 3
+    >>> x = 3.0 # derivative point
+    >>> h = 0.1 # differential step
+    >>> Np = 3 # point number for central derivative
+    >>> weights = _central_diff_weights(Np) # weights for first derivative
+    >>> vals = [f(x + (i - Np/2) * h) for i in range(Np)]
+    >>> sum(w * v for (w, v) in zip(weights, vals))/h
+    11.79999999999998
+
+    This value is close to the analytical solution:
+    f'(x) = 4x, so f'(3) = 12
+
+    References
+    ----------
+    .. [1] https://en.wikipedia.org/wiki/Finite_difference
+
+    """
+    if Np < ndiv + 1:
+        raise ValueError(
+            "Number of points must be at least the derivative order + 1."
+        )
+    if Np % 2 == 0:
+        raise ValueError("The number of points must be odd.")
+
+    ho = Np >> 1
+    x = arange(-ho, ho + 1.0)
+    x = x[:, newaxis]
+    X = x**0.0
+    for k in range(1, Np):
+        X = hstack([X, x**k])
+    w = prod(arange(1, ndiv + 1), axis=0) * linalg.inv(X)[ndiv]
+    return w
+
+
+def _derivative(func, x0, dx=1.0, n=1, args=(), order=3):
+    """
+    Find the nth derivative of a function at a point.
+
+    Given a function, use a central difference formula with spacing `dx` to
+    compute the nth derivative at `x0`.
+
+    Parameters
+    ----------
+    func : function
+        Input function.
+    x0 : float
+        The point at which the nth derivative is found.
+    dx : float, optional
+        Spacing.
+    n : int, optional
+        Order of the derivative. Default is 1.
+    args : tuple, optional
+        Arguments
+    order : int, optional
+        Number of points to use, must be odd.
+
+    Notes
+    -----
+    Decreasing the step size too small can result in round-off error.
+
+    Examples
+    --------
+    >>> def f(x):
+    ...     return x**3 + x**2
+    >>> _derivative(f, 1.0, dx=1e-6)
+    4.9999999999217337
+
+    """
+    if order < n + 1:
+        raise ValueError(
+            "'order' (the number of points used to compute the derivative), "
+            "must be at least the derivative order 'n' + 1."
+        )
+    if order % 2 == 0:
+        raise ValueError(
+            "'order' (the number of points used to compute the derivative) "
+            "must be odd."
+        )
+    # pre-computed for n=1 and 2 and low-order for speed.
+    if n == 1:
+        if order == 3:
+            weights = array([-1, 0, 1]) / 2.0
+        elif order == 5:
+            weights = array([1, -8, 0, 8, -1]) / 12.0
+        elif order == 7:
+            weights = array([-1, 9, -45, 0, 45, -9, 1]) / 60.0
+        elif order == 9:
+            weights = array([3, -32, 168, -672, 0, 672, -168, 32, -3]) / 840.0
+        else:
+            weights = _central_diff_weights(order, 1)
+    elif n == 2:
+        if order == 3:
+            weights = array([1, -2.0, 1])
+        elif order == 5:
+            weights = array([-1, 16, -30, 16, -1]) / 12.0
+        elif order == 7:
+            weights = array([2, -27, 270, -490, 270, -27, 2]) / 180.0
+        elif order == 9:
+            weights = (
+                array([-9, 128, -1008, 8064, -14350, 8064, -1008, 128, -9])
+                / 5040.0
+            )
+        else:
+            weights = _central_diff_weights(order, 2)
+    else:
+        weights = _central_diff_weights(order, n)
+    val = 0.0
+    ho = order >> 1
+    for k in range(order):
+        val += weights[k] * func(x0 + (k - ho) * dx, *args)
+    return val / prod((dx,) * n, axis=0)