def mfpt(T, target, origin=None, tau=1, mu=None):
    r"""Mean first passage times (from a set of starting states - optional)
    to a set of target states.

    Parameters
    ----------
    T : ndarray or scipy.sparse matrix, shape=(n,n)
        Transition matrix.
    target : int or list of int
        Target states for mfpt calculation.
    origin : int or list of int (optional)
        Set of starting states.
    tau : int (optional)
        The time-lag (in elementary time steps of the microstate
        trajectory) at which the given transition matrix was
        constructed.
    mu : (n,) ndarray (optional)
        The stationary distribution of the transition matrix T.

    Returns
    -------
    m_t : ndarray, shape=(n,) or shape(1,)
        Mean first passage time or vector of mean first passage times.

    Notes
    -----
    The mean first passage time :math:`\mathbf{E}_x[T_Y]` is the expected
    hitting time of one state :math:`y` in :math:`Y` when starting in state :math:`x`.

    For a fixed target state :math:`y` it is given by

    .. math :: \mathbb{E}_x[T_y] = \left \{  \begin{array}{cc}
                                             0 & x=y \\
                                             1+\sum_{z} T_{x,z} \mathbb{E}_z[T_y] & x \neq y
                                             \end{array}  \right.

    For a set of target states :math:`Y` it is given by

    .. math :: \mathbb{E}_x[T_Y] = \left \{  \begin{array}{cc}
                                             0 & x \in Y \\
                                             1+\sum_{z} T_{x,z} \mathbb{E}_z[T_Y] & x \notin Y
                                             \end{array}  \right.

    The mean first passage time between sets, :math:`\mathbf{E}_X[T_Y]`, is given by

    .. math :: \mathbb{E}_X[T_Y] = \sum_{x \in X}
                \frac{\mu_x \mathbb{E}_x[T_Y]}{\sum_{z \in X} \mu_z}

    References
    ----------
    .. [1] Hoel, P G and S C Port and C J Stone. 1972. Introduction to
        Stochastic Processes.

    Examples
    --------

    >>> import numpy as np
    >>> from msmtools.analysis import mfpt

    >>> T = np.array([[0.9, 0.1, 0.0], [0.5, 0.0, 0.5], [0.0, 0.1, 0.9]])
    >>> m_t = mfpt(T, 0)
    >>> m_t
    array([  0.,  12.,  22.])

    """
    # check inputs
    T = _types.ensure_ndarray_or_sparse(T, ndim=2, uniform=True, kind='numeric')
    target = _types.ensure_int_vector(target)
    origin = _types.ensure_int_vector_or_None(origin)
    # go
    if _issparse(T):
        if origin is None:
            t_tau = sparse.mean_first_passage_time.mfpt(T, target)
        else:
            t_tau = sparse.mean_first_passage_time.mfpt_between_sets(T, target, origin, mu=mu)
    else:
        if origin is None:
            t_tau = dense.mean_first_passage_time.mfpt(T, target)
        else:
            t_tau = dense.mean_first_passage_time.mfpt_between_sets(T, target, origin, mu=mu)

    # scale answer by lag time used.
    return tau * t_tau
r"""Mean first passage times (from a set of starting states - optional)
    to a set of target states.

    Parameters
    ----------
    T : ndarray or scipy.sparse matrix, shape=(n,n)
        Transition matrix.
    target : int or list of int
        Target states for mfpt calculation.
    origin : int or list of int (optional)
        Set of starting states.
    tau : int (optional)
        The time-lag (in elementary time steps of the microstate
        trajectory) at which the given transition matrix was
        constructed.
    mu : (n,) ndarray (optional)
        The stationary distribution of the transition matrix T.

    Returns
    -------
    m_t : ndarray, shape=(n,) or shape(1,)
        Mean first passage time or vector of mean first passage times.

    Notes
    -----
    The mean first passage time :math:`\mathbf{E}_x[T_Y]` is the expected
    hitting time of one state :math:`y` in :math:`Y` when starting in state :math:`x`.

    For a fixed target state :math:`y` it is given by

    .. math :: \mathbb{E}_x[T_y] = \left \{  \begin{array}{cc}
                                             0 & x=y \\
                                             1+\sum_{z} T_{x,z} \mathbb{E}_z[T_y] & x \neq y
                                             \end{array}  \right.

    For a set of target states :math:`Y` it is given by

    .. math :: \mathbb{E}_x[T_Y] = \left \{  \begin{array}{cc}
                                             0 & x \in Y \\
                                             1+\sum_{z} T_{x,z} \mathbb{E}_z[T_Y] & x \notin Y
                                             \end{array}  \right.

    The mean first passage time between sets, :math:`\mathbf{E}_X[T_Y]`, is given by

    .. math :: \mathbb{E}_X[T_Y] = \sum_{x \in X}
                \frac{\mu_x \mathbb{E}_x[T_Y]}{\sum_{z \in X} \mu_z}

    References
    ----------
    .. [1] Hoel, P G and S C Port and C J Stone. 1972. Introduction to
        Stochastic Processes.

    Examples
    --------

    >>> import numpy as np
    >>> from msmtools.analysis import mfpt

    >>> T = np.array([[0.9, 0.1, 0.0], [0.5, 0.0, 0.5], [0.0, 0.1, 0.9]])
    >>> m_t = mfpt(T, 0)
    >>> m_t
    array([  0.,  12.,  22.])