where a(>r) and σ are positive constants represent the expected
instantaneous rate of return of the risky asset and the volatility of
the risky asset price, respectively, and Wt
is a standard Brownian
motion.
Let At > 0 be the total amount of money invested in the risky
asset at time t under an investment strategy A. So, the associated
surplus process Xt
is the wealth of the insurer at time t if he adopts
strategy A and q. Since any amount not invested in the risky asset
is held in the risk-free asset, the surplus takes the form
dXt = At
dPt
Pt
+ (Xt − At)
dRt
Rt
+ (c − δ(qt))dt − qtdSt
= [rXt + (a − r)At + (c − δ(qt))]dt
+ Atσ dWt − qtdSt
. (2.3)
Remark 2.1. In this paper, we assume that continuous trading is
allowed, and that all assets are infinitely divisible. Also, we work on
a complete probability space (Ω, F , P) on which the process Xt
is
well defined. The information at time t is given by the complete filtration
Ft generated by stock price process Pt and aggregate claim
process St
. The set of all admissible strategies is denoted by Π, and
composed of strategies πt = (At, qt) which are Ft -predictable, and
satisfy the condition that E[
T
0
A
2
s
ds] < ∞ a.s., for all T < ∞.