add numerical recipes library

lib/nr/k_and_r/recipes/stoerm.c

@@ -0,0 +1,35 @@
+
+#include "nrutil.h"
+
+void stoerm(y,d2y,nv,xs,htot,nstep,yout,derivs)
+float d2y[],htot,xs,y[],yout[];
+int nstep,nv;
+void (*derivs)();
+{
+	int i,n,neqns,nn;
+	float h,h2,halfh,x,*ytemp;
+
+	ytemp=vector(1,nv);
+	h=htot/nstep;
+	halfh=0.5*h;
+	neqns=nv/2;
+	for (i=1;i<=neqns;i++) {
+		n=neqns+i;
+		ytemp[i]=y[i]+(ytemp[n]=h*(y[n]+halfh*d2y[i]));
+	}
+	x=xs+h;
+	(*derivs)(x,ytemp,yout);
+	h2=h*h;
+	for (nn=2;nn<=nstep;nn++) {
+		for (i=1;i<=neqns;i++)
+			ytemp[i] += (ytemp[(n=neqns+i)] += h2*yout[i]);
+		x += h;
+		(*derivs)(x,ytemp,yout);
+	}
+	for (i=1;i<=neqns;i++) {
+		n=neqns+i;
+		yout[n]=ytemp[n]/h+halfh*yout[i];
+		yout[i]=ytemp[i];
+	}
+	free_vector(ytemp,1,nv);
+}