“…The linear Black-Scholes equation with a constant volatility σ has been derived under several restrictive assumptions like e.g., frictionless, liquid and complete markets, etc. Such assumptions have been relaxed in order to model the presence of transaction costs (see e.g., Leland [25], Hoggard et al [19], Avellaneda and Paras [2]), feedback and illiquid market effects due to large traders choosing given stocktrading strategies (Frey [13], Frey and Patie [14], Frey and Stremme [15], Schönbucher and Wilmott [28]), imperfect replication and investor's preferences (Barles and Soner [4]), risk from unprotected portfolio (Kratka [22], Jandačka andŠevčovič [21] or [30]). …”