def get_frame_IDs(objects_archive, start, end, every):
    """
    Returns list of ID numbers of the objects identified in each frame.
    
    Parameters
    ----------
    objects_archive : dictionary
        Dictionary of objects identified in a video labeled by ID number
    start, end, every : ints
        start = index of first frame to analyze; end = index of last frame
        to analyze; every = analyze every `every` frame (e.g., if every = 3,
        analyzes every 3rd frame)
    
    Returns
    -------
    frame_IDs : dictionary
        Dictionary indexed by frame number in the video. Each entry is
        a list of the ID numbers of the objects identified in that frame.
    """
    # initializes dictionary of IDs for each frame
    frame_IDs = {}
    for f in range(start, end, every):
        frame_IDs[f] = []
    # loads IDs of objects found in each frame
    for ID in objects_archive.keys():
        obj = objects_archive[ID]
        frames = obj.get_props('frame')
        for f in frames:
            frame_IDs[f] += [ID]

    return frame_IDs