def get_betas(alpha, M, U):
    """get array with beta values for each fold

    Args:
        alpha (float): pseudo count
        M (int array): Number of mutated sites in each fold
        U (int array): Number of unmutated sites in each fold

    Returns:
        float array: array with beta value for each fold
    """
    my=M/(M+U)
    betas = (alpha*(1.0-my))/my
    return betas