import math


def point_on_sphere(r, s_deg, t_deg, center):
  """
  Returns a point on sphere of radius r: see definition of polar (sperical)
  coordinate system in 3D.
  """
  sin = math.sin
  cos = math.cos
  s = s_deg*math.pi/180
  t = t_deg*math.pi/180
  x = center[0] + r * cos(s) * sin(t)
  y = center[1] + r * sin(s) * sin(t)
  z = center[2] + r * cos(t)
  return x,y,z