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FX options in target zones

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Pages 1477-1486 | Received 21 Dec 2015, Accepted 14 Sep 2016, Published online: 23 Nov 2016
 

Acknowledgements

ZK would like to thank Eyal Neuman for discussions on Brownian motion with reflecting barriers, which prompted him to think about this topic. We would also like to thank Travis Fisher for discussions.

Notes

No potential conflict of interest was reported by the authors.

1 For a literature survey, see, e.g. (Duarte,Andrade and Duarte, Citation2013). For a partial list (with some related literature, including on option pricing), see, e.g. (Andersen,Bollerslev,Diebold and Labys, Citation2001), (Anthony and MacDonald, Citation1998), (Ayuso and Restoy, Citation1996), (Ball and Roma, Citation1994), (Bauer,De Grauwe and Reitz, Citation2009), (Beetsma and Van Der Ploeg, Citation1994), (Bekaert and Gray, Citation1998), (Bertola and Caballero, Citation1992), (Bertola and Svensson, Citation1993), (Black and Scholes, Citation1973), (Bo,Wang and Yang, Citation2011), (Bo,Li,Ren,Wang and Yang, Citation2011), (Campa and Chang, Citation1996), (Carr,Ellis and Gupta, Citation1998), (Carr and Jarrow, Citation1990), (Carr and Linetsky, Citation2000), (Cavaliere, Citation1998), (Chinn, Citation1991), (Cornell, Citation2003), (Christensen,Lando and Miltersen, Citation1998), (De Jong, Citation1994), (De Jong,Drost and Werker, Citation2001), (Delgado,Dumas, Citation1992), (Dominquez and Kenen, Citation1992), (Driffill and Sola, Citation2006), (Duarte,Andrade and Duarte, Citation2010), Dumas et al. (Citation1995a, Citationb), (Edin and Vredin, Citation1993), (Edison,Miller and Williamson, Citation1987), (Flood and Garber, Citation1991), (Flood,Rose and Mathieson, Citation1991), (Garman and Kohlhagen, Citation1983), (Grabbe, Citation1983), (Harrison, Citation1985), (Harrison and Pliska, Citation1981), (Honogan, Citation1998), (Hull and White, Citation1987), (Kempa and Nelles, Citation1999), (Klaster and Knot, Citation2002), (Klein and Lewis, Citation1993), (Koedijk,Stork and de Vries, Citation1998), Krugman (Citation1991, Citation1992), (Lai,Fang and Chang, Citation2008), (Larsen and Sørensen, Citation2007), (Lin, Citation2008), Lindberg and Söderlind (Citation1994a, Citationb), (Lindberg,Söderlind and Svensson, Citation1993), (Linetsky, Citation2005), (Lundbergh and Teräsvirta, Citation2006), (Magnier, Citation1992), McKinnon (Citation1982, Citation1984), Meese and Rose (Citation1990, Citation1991), Merton (Citation1973, Citation1976), (Miller and Weller, Citation1991), (Mizrach, Citation1995), (Obstfeld and Rogoff, Citation1995), (Rangvid and Sørensen, Citation2001), (Rose and Svensson, Citation1995), (Saphores, Citation2005), (Serrat, Citation2000), (Slominski, Citation1994), (Smith,Spencer, Citation1992), (Sutherland, Citation1994), Svensson (Citation1991a, Citationb, Citation1992a, Citationb, Citation1993, Citation1994), (Taylor and Iannizzotto, Citation2001), Torres (Citation2000a, Citationb), (Tronzano,Psaradakis and Sola, Citation2003), (Veestraeten, Citation2008), (Vlaar and Palm, Citation1993), (Ward and Glynn, Citation2003), (Werner, Citation1995), Williamson (Citation1985, Citation1986, Citation1987a, Citationb, Citation1989, Citation2002), (Williamson and Miller, Citation1987), (Yu, Citation2007), (Zhang, Citation1994), (Zhu, Citation1996), and references therein.

2 E.g. the USD/HKD spread trades between 7.75 and 7.85, a band fixed by the Hong Kong Monetary Authority.

3 More generally, we can assume that any volatility in the domestic bond is uncorrelated with the volatility in the FX rate and the volatility in the foreign bond , or, more precisely, any such correlation is negligible at relevant time horizons. This would not alter any of the subsequent discussions or conclusions, so for the sake of simplicity we will assume that is deterministic.

4 Alternatively, we can set to zero, so is the same as the foreign cash bond , and restore the (deterministic) dependence at the end by multiplying all derivative prices by .

5 FX has an analog in equities. Consider a stock with a continuous dividend rate . Then the risk-free interest rate is analogous to the domestic short-rate, the dividend rate is analogous to the foreign short-rate, and the stock is analogous to the foreign currency (so the stock price is analogous to the worth of one unit of the foreign currency in terms of the domestic currency).

6 I.e. the probability density of starting from and ending at , where .

7 In the log-normal Black–Scholes model the discounted process is given by (Equation11).

8 We consider time-homogeneous dynamics so the problem is analytically tractable (see below).

9 With the view, e.g. to have a function with attainable barriers at . Also, note that, unless , this is not the same as having time-independent barriers for .

10 I.e. (i) is a local function of and t, and (ii) is independent of the history .

11 Thus, Dirichlet or Robin boundary conditions would be inconsistent with being a martingale. See appendix 2 for the transition density and martingales for Robin boundary conditions.

12 Monotonicity is assumed so we can price claims (see below).

13 Otherwise, barring any contrived time dependence, generically will break the band.

14 For constant () we would have minima (maxima) at and with a maximum (minimum) located between and , which is not possible for on .

15 We assume that U(x) is bounded on , so the spectrum is bounded from below.

16 Notice that by virtue of (Equation32), hence (Equation31) for as .

17 Indeed, from (Equation31) with and and the fact that , it follows that must flip sign on , i.e. has at least one node. However, if any , then corresponding to would have to have at least one node, which is not possible.

18 The actual hedge (recall that we are operating from the foreign investor’s perspective) consists of holding units of the domestic cash bond, and units of the foreign cash bond (which is the foreign investor’s numeraire). As mentioned above, if we set the domestic short-rate to zero, (the worth of 1 unit of the domestic cash bond in the foreign currency) becomes a foreign tradable.

19 This is where the monotonicity of f(x) is important.

20 In the model (Equation46) we have , where , so .

21 In fact, here we assume that is not too close to or else the drift becomes large near the boundaries. In the limit the boundaries are no longer attainable.

22 Near the drift is approximately linear as in the OU process: ; however, away from the long-run value (i.e. L / 2), the nonlinear effects become important. Unlike the OU case with reflecting boundaries, the model (Equation61) is solvable via elementary functions.

23 The shift is due to the log-normal form of the FX rate. E.g. if , the expectation .

24 If the domestic short-rate is low, , then the foreign short-rate would become negative for . Theoretically this would appear to imply arbitrage. However, in practice the short-rate is not a tradable instrument and this situation may not be arbitrageable as the tradable bonds (for the actually available maturities) may still have well-behaved yields despite the negative underlying short-rate (for a recent discussion, see, e.g. (Kakushadze, Citation2016) and references therein). Also note that (Equation66) simply follows from and (Equation45), where is the forward rate.

25 And is continuous even if U(x) contains -functions, i.e. when is discontinuous.

26 Hence our choice of a constant diffusion coefficient . When the band is narrow, there is little benefit to having nonconstant as the boundaries are reflecting. This is to be contrasted with the case of unattainable boundaries where tends to zero near the boundaries.

28 Usually, the numeraire is chosen to be a cash bond, but it can be any tradable instrument.

29 Otherwise, the market would be incomplete and we would not be able to hedge claims.

30 Here, more generally, we can impose different Robin boundary conditions at and . For our purposes here it will suffice to consider (EquationB5). Let us mention that, in the case of different boundary conditions the spectrum generally has an infinite tower of positive eigenvalues, and also two additional eigenvalues, at least one of which is negative. (More precisely, there are non-generic degenerate cases with only one such additional eigenvalue, which is negative.)

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