def stochastic_string(reactant_name, number):
    """
        This function returns the stochastic string expression for mass action kinetics
        For instance the reaction 2A -> 3A would imply A*(A-1)/2
        It only does so for one reactant, so it must be called for all reactants in the reaction

        Parameters:
            reactant_name (str) = species string involved in the reaction
            number (int) = stoichiometry (number of times it appears)

        Returns:
            to_return_string (str) = the mass action kinetics string expression for only that species
    """
    to_return_string = ''
    for i in range(number):
        if i == 0:
            to_return_string = reactant_name
        else:
            to_return_string += f' * ({reactant_name} - {i})/{i + 1}'

    return to_return_string