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/* -*- mode: c++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */

 Copyright (C) 2001, 2002, 2003 Sadruddin Rejeb

 This file is part of QuantLib, a free-software/open-source library
 for financial quantitative analysts and developers - http://quantlib.org/

 QuantLib is free software: you can redistribute it and/or modify it
 under the terms of the QuantLib license.  You should have received a
 copy of the license along with this program; if not, please email
 <quantlib-dev@lists.sf.net>. The license is also available online at

 This program is distributed in the hope that it will be useful, but WITHOUT
 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 FOR A PARTICULAR PURPOSE.  See the license for more details.

/*! \file blackkarasinski.hpp
    \brief Black-Karasinski model

#ifndef quantlib_one_factor_models_black_karasinski_h
#define quantlib_one_factor_models_black_karasinski_h

#include <ql/ShortRateModels/onefactormodel.hpp>
#include <ql/Processes/ornsteinuhlenbeckprocess.hpp>

namespace QuantLib {

    //! Standard Black-Karasinski model class.
    /*! This class implements the standard Black-Karasinski model defined by
            d\ln r_t = (\theta(t) - \alpha \ln r_t)dt + \sigma dW_t,
        where \f$ alpha \f$ and \f$ sigma \f$ are constants.

        \ingroup shortrate
00041     class BlackKarasinski : public OneFactorModel,
                            public TermStructureConsistentModel {
        BlackKarasinski(const Handle<YieldTermStructure>& termStructure,
                        Real a = 0.1, Real sigma = 0.1);

00047         boost::shared_ptr<ShortRateDynamics> dynamics() const {
            QL_FAIL("no defined process for Black-Karasinski");

        boost::shared_ptr<Lattice> tree(const TimeGrid& grid) const;

        class Dynamics;
        class Helper;

        Real a() const { return a_(0.0); }
        Real sigma() const { return sigma_(0.0); }

        Parameter& a_;
        Parameter& sigma_;

    //! Short-rate dynamics in the Black-Karasinski model
    /*! The short-rate is here
            r_t = e^{\varphi(t) + x_t}
         where \f$ \varphi(t) \f$ is the deterministic time-dependent
         parameter (which can not be determined analytically)
         used for term-structure fitting and \f$ x_t \f$ is the state
         variable following an Ornstein-Uhlenbeck process.
00074     class BlackKarasinski::Dynamics
        : public BlackKarasinski::ShortRateDynamics {
        Dynamics(const Parameter& fitting, Real alpha, Real sigma)
        : ShortRateDynamics(boost::shared_ptr<StochasticProcess>(
                                 new OrnsteinUhlenbeckProcess(alpha, sigma))),
          fitting_(fitting) {}

00082         Real variable(Time t, Rate r) const {
            return std::log(r) - fitting_(t);

00086         Real shortRate(Time t, Real x) const {
            return std::exp(x + fitting_(t));
        Parameter fitting_;



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