def minimum_mutation_frequency(M, N, a):
    """
    # ========================================================================

    MINIMUM MUTATION FREQUENCY


    PURPOSE
    -------

    Calculates the minimum mutation frequency.


    INPUT
    -----

    [INT] [M]
        The number of mutations.

    [INT] [N]
        The total number of clones (reads) sampled from the viral
        quasispecies.

    [INT] [a]
        The length of the amplicons.


    RETURN
    ------

    [FLOAT]
        The minimum mutation frequency.

    # ========================================================================
    """

    Mfmin = float(M) / (float(N) * float(a))

    return Mfmin