Backward Stochastic Differential Equations With Continuous Coefficient
Abstract
In this paper, we investigate backward stochastic differential equations driven by G-Brownian motion with uniformly continuous coefficients in (y,z). The existence and uniqueness of solutions are obtained via a method of Picard iteration, a linearization method and a monotone convergence argument. Furthermore, we establish the corresponding comparison theorem and related nonlinear Feynman–Kac formula.
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Appendix: Comparison Theorem for PDE (4.7)
Appendix: Comparison Theorem for PDE (4.7)
Theorem A.1
Suppose assumptions (A1)–(A3) hold. Let \(u^1\) be a viscosity subsolution and \(u^2\) be a viscosity supersolution to PDE (4.7) satisfying the polynomial growth condition, respectively. Then, \(u^1\le u^2\) on \([0, T]\times \mathbb {R}^{n}\) provided that \(u^1|_{t=T}\le u^2|_{t=T}\).
By doing the transformation \(v(t,x):=u(T-t,x)\), we know that if we want to get the comparison theorem A.1 for PDE (4.7), we only need to prove the following comparison theorem A.2 for PDE
$$\begin{aligned} \left\{ \begin{array}{ll} &{}\partial _{t}v-F(t,x,v(t,x),D_{x}v,D^{2}_{x}v)=0\\ &{} v(0,x)=\phi (x),\\ \end{array} \right. \end{aligned}$$
(A.1)
where F is defined by (4.8). For the definition of the viscosity solution to PDE (A.1), see Definition 1.1 in Appendix C of [13].
Theorem A.2
Suppose assumptions (A1)–(A3) hold. Let \(v^1\) be a viscosity subsolution and \(v^2\) be a viscosity supersolution to PDE (A.1) satisfying the polynomial growth condition, respectively. Then, \(v^1\le v^2\) on \([0, T]\times \mathbb {R}^{n}\) provided that \(v^1|_{t=0}\le v^2|_{t=0}\).
Proof
The main idea comes from Theorem 2.2 in Appendix C of [13]. For reader's convenience, we give a brief proof.
step1.
For a large constant \(\lambda >0 \) to be chosen in step 2, we set \(\eta (x):=(1+|x|^{2})^{l/2}\) and
$$\begin{aligned} v_1(t,x):=v^1(t,x)\eta ^{-1} (x)e^{-\lambda t}, \ v_2(t,x):=v^2(t,x)\eta ^{-1} (x)e^{-\lambda t}, \end{aligned}$$
where \(l\ge 2\) is chosen to be large enough such that \(|v_{1}|+|v_{2}|\rightarrow 0\) uniformly as \(|x|\rightarrow \infty \). It is easy to verify that, for \(i=1,2\), \(v_{i}\) is a bounded viscosity subsolution of
$$\begin{aligned} \partial _{t}v_{i}+\lambda v_i-F^*_i(t,x,v _{i},Dv_i,D^{2}v_{i})=0, \end{aligned}$$
where the function \(F^*_1(t,x,v,p,X)=F^*(t,x,v,p,X), F^*_2(t,x,v,p,X)=-F^*(t,x,-v,-p,-X)\) and
$$\begin{aligned}&F^*(t,x,v,p,X)\\&\quad :=e^{-\lambda t}\eta ^{-1}(x)F(t,x,e^{\lambda t}v\eta (x),e^{\lambda t}(p\eta (x)+vD\eta (x) ),e^{\lambda t}(X\eta (x)\\&\qquad +p\otimes D\eta (x) +D\eta (x) \otimes p+vD^{2}\eta (x) )) \end{aligned}$$
for any \((x,v,p,X)\in \mathbb {R}^n\times \mathbb {R}\times \mathbb {R}^n\times \mathbb {S}_{n}\). Here, \(p\otimes D\eta (x)=(p^iD_{x_{j}}\eta )^{n}_{i,j=1}\) and
$$\begin{aligned} D\eta (x)&=l \eta (x)(1+|x|^{2})^{-1}x,\ \ \ \ \\ \ D^{2}\eta (x)&=\eta (x)[l(1+|x|^{2})^{-1}I_{n}+l(l-2)(1+|x|^{2})^{-2}x\otimes x]. \end{aligned}$$
Obviously, for \(l\ge 2\), \(\eta ^{-1}(x)D\eta (x)\) and \(\eta ^{-1}(x)D^{2}\eta (x)\) are uniformly bounded functions.
Now, prove that \(v_{1}+v_{2}\le 0\). According to the proof of Theorem 2.2 in [13], it suffices to prove the conclusion under the additional assumptions: for each \(\bar{\delta }>0\),
$$\begin{aligned} \partial _{t}v_{i}+\lambda v_{i} -F^*_{i}(t,x,v_{i},Dv_{i},D^{2}v_{i})\le -\bar{\delta }/T^{2},\ \text {and }\lim _{t\rightarrow T}v_{i}(t,x)=-\infty \ \text {uniformly on } \mathbb {R}^{n}. \end{aligned}$$
(A.2)
Assume the contrary that
$$\begin{aligned} \sup _{(t,x)\in [0,T)\times \mathbb {R}^{n}}(v_{1}(t,x)+v_{2}(t,x))>0. \end{aligned}$$
Notice that \(\left( v_1(t,x)+v_2(t,x)\right) ^+\rightarrow 0\) uniformly as \(|x|\rightarrow \infty \). Then, according to the proof of Theorem 2.2 in [13], for large enough \(\alpha >0\), there exists some point \((t^{\alpha },x_{1}^{\alpha },x_{2}^{\alpha })\) inside a compact subset of \([0,T)\times \mathbb {R}^{2n}\) such that \(v_{1}(t^{\alpha },x_{1}^{\alpha })+v_{2}(t^{\alpha },x_{2}^{\alpha })-\frac{\alpha }{2}|x_{1}^{\alpha }-x_{2}^{\alpha }|^2>0\) and
$$\begin{aligned} \ \lim _{\alpha \rightarrow \infty }\alpha |x_{1}^{\alpha }-x_{2}^{\alpha }|^2=0, \ \text {and}\ \lim _{\alpha \rightarrow \infty }(t^\alpha ,x_{1}^{\alpha },x_{2}^{\alpha })=(\hat{t},\hat{x},\hat{x})\ \hbox { for some}\ (\hat{t},\hat{x})\in (0,T)\times \mathbb {R}^n. \end{aligned}$$
Then, there exist \(b_{i}^{\alpha }\in \mathbb {R}\), \(X_{i}^{\alpha }\in \mathbb {S}_{n}\) such that \(b_1^{\alpha }+b^{\alpha }_2=0\),
$$\begin{aligned} (b_{1}^{\alpha },p_{1}^{\alpha },X_{1}^{\alpha } )\in \mathcal {\bar{P}}^{2,+}v_{1}(t^{\alpha },x_{1}^{\alpha }), \ \ (b_{2}^{\alpha },p_{2}^{\alpha },X_{2}^{\alpha } )\in \mathcal {\bar{P}}^{2,+}v_{2}(t^{\alpha },x_{2}^{\alpha }), \end{aligned}$$
and
$$\begin{aligned} \left( \begin{array}[c]{cccc} X_1^{\alpha } &{} 0\\ 0 &{} X_2^{\alpha } \end{array} \right) \le A+\frac{1}{\alpha }A^{2}, \ A=\alpha \left( \begin{array}[c]{cccc} I_n &{} -I_n\\ -I_n &{} I_n \end{array}\right) , \end{aligned}$$
where \(p_{1}^{\alpha }=\alpha (x_{1}^{\alpha }-x_{2}^{\alpha }), p_{2}^{\alpha }=\alpha (x_{2}^{\alpha }-x_{1}^{\alpha }).\) Obviously, \(p_{1}^{\alpha }+p_{2}^{\alpha }=0\), \( \lim _{\alpha \rightarrow \infty }|p_{i}^{\alpha }||x_{1}^{\alpha }-x_{2}^{\alpha }|=0\) and
$$\begin{aligned} \left( \begin{array}[c]{cccc} X_1^{\alpha } &{} 0\\ 0 &{} X_2^{\alpha } \end{array} \right) \le 3\alpha \left( \begin{array}[c]{cccc} I_n &{} -I_n\\ -I_n &{} I_n \end{array}\right) . \end{aligned}$$
(A.3)
Moreover, it follows from Eq. (A.2) that
$$\begin{aligned}&b_{1}^{\alpha }+\lambda v_{1}(t^\alpha ,x_1^{\alpha }) -F^*_{1}(t^{\alpha },x_1^{\alpha },v_{1}(t^\alpha ,x_1^{\alpha }),p_{1}^{\alpha },X_1^{\alpha })\le -\bar{\delta }/T^{2},\\&b_{2}^{\alpha }+\lambda v_{2}(t^\alpha ,x_2^{\alpha }) -F^*_{2}(t^{\alpha },x_2^{\alpha },v_{2}(t^\alpha ,x_2^{\alpha }),p_{2}^{\alpha },X_2^{\alpha })\le -\bar{\delta }/T^{2}. \end{aligned}$$
According to the definition of \(F^*_i\), we derive that
$$\begin{aligned} -2\bar{\delta }/T^{2}&\ge \lambda v_{2}(t^{\alpha },x_{2}^{\alpha })+F^*(t^{\alpha },x_{2}^{\alpha },-v_{2}(t^{\alpha },x_{2}^{\alpha }),p_{1}^{\alpha },-X_{2}^{\alpha })\\&\quad +\lambda v_{1}(t^{\alpha },x_{1}^{\alpha })-F^*(t^{\alpha },x_{1}^{\alpha },v_{1}(t^{\alpha },x_{1}^{\alpha }),p_{1}^{\alpha },X_{1}^{\alpha }),\\&:=I_{1}^{\alpha }+I_{2}^{\alpha }, \end{aligned}$$
where
$$\begin{aligned} I_{1}^{\alpha }&:=\lambda v_{2}(t^{\alpha },x_{2}^{\alpha })+F^*(t^{\alpha },x_{2}^{\alpha },-v_{2}(t^{\alpha },x_{2}^{\alpha }),p_{1}^{\alpha },-X_{2}^{\alpha }) +\lambda v_{1}(t^{\alpha },x_{1}^{\alpha })\\&\quad -F^*(t^{\alpha },x_{2}^{\alpha },v_{1}(t^{\alpha },x_{1}^{\alpha }),p_{1}^{\alpha },-X_{2}^{\alpha })\\ I_{2}^{\alpha }&:=F^*(t^{\alpha },x_{2}^{\alpha },v_{1}(t^{\alpha },x_{1}^{\alpha }),p_{1}^{\alpha },-X_{2}^{\alpha })- F^*(t^{\alpha },x_{1}^{\alpha },v_{1}(t^{\alpha },x_{1}^{\alpha }),p_{1}^{\alpha },X_{1}^{\alpha }). \end{aligned}$$
We claim that
$$\begin{aligned} I_{1}^{\alpha }\ge 0\ \text { as } \ \alpha \rightarrow \infty , \end{aligned}$$
(A.4)
and
$$\begin{aligned} I_{2}^{\alpha }\ge 0\ \text {as} \ \alpha \rightarrow \infty , \end{aligned}$$
(A.5)
whose proof will be given in step 2 and step 3, respectively. Then, we induce a contradiction. Consequently, we get that \(v^1\le v^2.\) The proof is complete.
step2. The proof of (A.4).
Note that \(|G(A)|\le \frac{1}{2}\bar{\sigma }^{2}\sqrt{d}|A|, \forall A\in \mathbb {S}_{d}.\) For large enough \(\alpha ,\) any \(v\ge \tilde{v}\), by the definition of \(F^*\), assumptions (A2), (A3) and Remark 4.1, we can get
$$\begin{aligned}&F^*(t^{\alpha },x_{2}^{\alpha },v,p_{1}^{\alpha },-X_{2}^{\alpha })-F^*(t^{\alpha },x_{2}^{\alpha },\tilde{v},p_{1}^{\alpha },-X_{2}^{\alpha })\\&\quad \le C[(v-\tilde{v})+\rho (C(v-\tilde{v}))+\psi (C(v-\tilde{v}))]\\&\quad \le C+C(v-\tilde{v}), \end{aligned}$$
where the constant C is only dependent on \(L,K,\bar{\sigma },d,T.\) Then, we have
$$\begin{aligned}&-\lambda v+F^*(t^{\alpha },x_{2}^{\alpha },v,p_{1}^{\alpha },-X_{2}^{\alpha })+\lambda \tilde{v}-F^*(t^{\alpha },x_{2}^{\alpha },\tilde{v},p_{1}^{\alpha },-X_{2}^{\alpha })\\&\quad \le -(\lambda -C)(v-\tilde{v})+C. \end{aligned}$$
By virtue of the boundness of the viscosity solution v and \(\tilde{v}\), we can choose \(\lambda \) large enough, so that the function
$$\begin{aligned} v\rightarrow -\lambda v+F^*(t^{\alpha },x_{2}^{\alpha },v,p_{1}^{\alpha },-X_{2}^{\alpha })\ \text {is non-increasing.} \end{aligned}$$
In view of \(-v_{2}(t^{\alpha },x_{2}^{\alpha })<v_{1}(t^{\alpha },x_{1}^{\alpha })\), (A.4) is proved.
step3. The proof of (A.5).
Note that \(G(A)\le \frac{1}{2}\overline{\sigma }^2\mathrm {tr}[A]\) for any \(A\ge 0\). By virtue of (A.3) and assumption (A2), we can get
$$\begin{aligned}&G(\sigma ^{T}(t^{\alpha },x_{1}^{\alpha })X_{1}^{\alpha }\sigma (t^{\alpha },x_{1}^{\alpha })-\sigma ^{T}(t^{\alpha },x_{2}^{\alpha })(-X_{2}^{\alpha })\sigma (t^{\alpha },x_{2}^{\alpha }))\\&\quad =G\left( \left( \begin{array}[c]{cccc} \sigma ^{T}(t^{\alpha },x_{1}^{\alpha }) &{} \sigma ^{T}(t^{\alpha },x_{2}^{\alpha })\\ \end{array} \right) \left( \begin{array}[c]{cccc} X_1^{\alpha } &{} 0\\ 0 &{} X_2^{\alpha } \end{array} \right) \left( \begin{array}[c]{cccc} \sigma (t^{\alpha },x_{1}^{\alpha }) \\ \sigma (t^{\alpha },x_{2}^{\alpha }) \end{array} \right) \right) \\&\quad \le 3\alpha G\left( \left( \begin{array}[c]{cccc} \sigma ^{T}(t^{\alpha },x_{1}^{\alpha }) &{} \sigma ^{T}(t^{\alpha },x_{2}^{\alpha })\\ \end{array} \right) \left( \begin{array}[c]{cccc} I_{n} &{} -I_{n}\\ -I_{n} &{} I_{n} \end{array} \right) \left( \begin{array}[c]{cccc} \sigma (t^{\alpha },x_{1}^{\alpha }) \\ \sigma (t^{\alpha },x_{2}^{\alpha }) \end{array} \right) \right) \\&\quad =3\alpha G\left( (\sigma (t^{\alpha },x_{1}^{\alpha })-\sigma (t^{\alpha },x_{2}^{\alpha }))^{T}(\sigma (t^{\alpha },x_{1}^{\alpha })-\sigma (t^{\alpha },x_{2}^{\alpha })\right) \\&\quad \le \frac{3}{2}\bar{\sigma }^{2}\alpha |\sigma (t^{\alpha },x_{1}^{\alpha })-\sigma (t^{\alpha },x_{2}^{\alpha })|^{2}\\&\quad \le \frac{3}{2}\bar{\sigma }^{2}L^{2}\alpha |x_{1}^{\alpha }-x_{2}^{\alpha }|^{2}. \end{aligned}$$
Note that \(|G(A)|\le \frac{1}{2}\bar{\sigma }^{2}\sqrt{d}|A|, \forall A\in \mathbb {S}_{d}.\) According to the definition of \(F^*\), assumptions (A2), (A3) and Remark 4.1, we can obtain
$$\begin{aligned}&I_{2}^{\alpha }\ge -C[|x_{1}^{\alpha }-x_{2}^{\alpha }|+|p_{1}^{\alpha }||x_{1}^{\alpha }-x_{2}^{\alpha }|+|v_{1}(t^{\alpha },x_{1}^{\alpha })||x_{1}^{\alpha }-x_{2}^{\alpha }|\\&\quad +\rho (C|v_{1}(t^{\alpha },x_{1}^{\alpha })||x_{1}^{\alpha }-x_{2}^{\alpha }|) +\psi (C(|p_{1}^{\alpha }||x_{1}^{\alpha }-x_{2}^{\alpha }|+|v_{1}(t^{\alpha },x_{1}^{\alpha })||x_{1}^{\alpha }-x_{2}^{\alpha }|))\\&\quad +\alpha |x_{1}^{\alpha }-x_{2}^{\alpha }|^{2}], \end{aligned}$$
where the constant C is only dependent on \(L,K,\bar{\sigma },d,T.\) Note that \( \lim _{\alpha \rightarrow \infty }|x_{1}^{\alpha }-x_{2}^{\alpha }|=0, \) \(\lim _{\alpha \rightarrow \infty }\) \(|v_{1}(t^\alpha ,x^\alpha )||x_{1}^{\alpha }-x_{2}^{\alpha }|=0,\) \(\lim _{\alpha \rightarrow \infty }|p_{1}^{\alpha }||x_{1}^{\alpha }-x_{2}^{\alpha }|=0, \lim _{\alpha \rightarrow \infty }\alpha |x_{1}^{\alpha }-x_{2}^{\alpha }|^{2}=0\) and \(\rho (\cdot )\) and \(\psi (\cdot )\) are continuous functions. Letting \(\alpha \rightarrow \infty \), we can get \(I_{2}^{\alpha }\ge 0.\) (A.5) is proved. \(\square \)
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Sun, S. Backward Stochastic Differential Equations Driven by G-Brownian Motion with Uniformly Continuous Coefficients in (y,z). J Theor Probab 35, 370–409 (2022). https://doi.org/10.1007/s10959-020-01057-2
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DOI : https://doi.org/10.1007/s10959-020-01057-2
Keywords
- G-Brownian motion
- G-BSDEs
- Comparison theorem
- Feynman–Kac formula
Mathematics Subject Classification (2020)
- 60H10
- 60H30
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