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diff --git a/fg21sim/utils/random.py b/fg21sim/utils/random.py
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+# Copyright (c) 2016 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Custom utilities of random number generations.
+"""
+
+import numpy as np
+
+
+def spherical_uniform(n=1):
+    """Uniformly pick random points on the surface of a unit sphere.
+    The algorithm is described in [SpherePointPicking]_.
+
+    Parameters
+    ----------
+    n : int
+        Number of points to be randomly picked
+
+    Returns
+    -------
+    theta : float, or 1D `~numpy.ndarray`
+        The polar angles, φ ∈ [0, π]. (unit: rad)
+        If ``n > 1``, then returns a 1D array containing all the generated
+        coordinates. (unit: rad)
+    phi : float, or 1D `~numpy.ndarray`
+        The azimuthal angles, θ ∈ [0, 2π).
+
+    NOTE
+    ----
+    Physicists usually adopt the (radial, polar, azimuthal) order with
+    the (r, θ, φ) notation for the spherical coordinates convention, which
+    is adopted here and by ``healpy``.
+    However, this convention is *different* to the convention generally
+    used by mathematicians.
+
+    References
+    ----------
+    .. [SpherePointPicking]
+       Wolfram MathWorld - Sphere Point Picking
+       http://mathworld.wolfram.com/SpherePointPicking.html
+
+    .. [SphericalCoordinates]
+       Wolfram MathWorld - Spherical Coordinates
+       http://mathworld.wolfram.com/SphericalCoordinates.html
+    """
+    u = np.random.uniform(size=n)
+    v = np.random.uniform(size=n)
+    phi = 2*np.pi * u
+    theta = np.arccos(2*v - 1)
+    return (theta, phi)