sfduffing (2.0-git)
Duffing differential equation solved by 4th order Runge-Kutta method.

        sfduffing < in.rsf > out.rsf sfile=sfile.rsf gamma=0.75 omega=1 kxi=1 x0=0 y0=0 pow1=1 pow2=3 verb=n ricker=n
Duffing equation: x'' + 0.5 x' - x + x^3 = gamma cos(omega t) + kxi input(t)

float gamma=0.75
strength of external force
float kxi=1
adjustment for input signal
float omega=1
angular frequence of external force
int pow1=1
power of first non-linear restitution term
int pow2=3
power of second non-linear restitution term
bool ricker=n [y/n]
if y need extenal input for external force
string sfile=
auxiliary input file name
bool verb=n [y/n]
verbosity flag
float x0=0
initial value of x0
float y0=0
initial value of y0