  Space discretization of some time-dependent partial differential equations
gives rise to systems of ordinary differential equations in additive form whose
terms have different stiffness properties. In these cases, implicit methods
should be used to integrate the stiff terms while efficient explicit methods
can be used for the non-stiff part of the problem. However, for systems with a
large number of equations, memory storage requirement is also an important
issue. When the high dimension of the problem compromises the computer memory
capacity, it is important to incorporate low memory usage to some other
properties of the scheme. In this paper we consider Additive Semi-Implicit
Runge-Kutta (ASIRK) methods, a class of implicitexplicit Runge-Kutta methods
for additive differential systems. We construct two second order 3-stage ASIRK
schemes with low-storage requirements. Having in mind problems with stiffness
parameters, besides accuracy and stability properties, we also impose stiff
accuracy conditions. The numerical experiments done show the advantages of the
new methods.
