  General formula for causal Green's function of linear differential operator
of given degree in one variable is given according to coefficient functions of
differential operator as a series of integrals. The solution also provides
analytic formula for fundamental solutions of corresponding homogenous linear
differential equation as series of integrals. Furthermore, multiplicative
property of causal Green's functions is shown and by which explicit formulas
for causal Green's functions of some classes of decomposable linear
differential operators are given. A method to find Green's function of general
linear differential operator of given degree in one variable with arbitrary
boundary condition according to coefficient functions of differential operator
is demonstrated.
