  Mathematical base of difference operators in Moving Particle Semi-implicit
method (MPS) are not given sufficiently in contrast to Smooth Particle
Hydrodynamics method (SPH). Iribe and Nakaza proposed a method to improve the
accuracy of the gradient operator, and Khayyer and Gotoh gave an ingenuity also
for gradient operator too. An extension to higher order difference operators of
Iribe-Nakaza method is given in this paper. The proposed method is a special
case of the author's method called Discrete Differential Operators on Irregular
Nodes (DDIN).
