add numerical recipes library

lib/nr/k_and_r/recipes/zbrak.c

@@ -0,0 +1,23 @@
+
+void zbrak(fx,x1,x2,n,xb1,xb2,nb)
+float (*fx)(),x1,x2,xb1[],xb2[];
+int *nb,n;
+{
+	int nbb,i;
+	float x,fp,fc,dx;
+
+	nbb=0;
+	dx=(x2-x1)/n;
+	fp=(*fx)(x=x1);
+	for (i=1;i<=n;i++) {
+		fc=(*fx)(x += dx);
+		if (fc*fp <= 0.0) {
+			xb1[++nbb]=x-dx;
+			xb2[nbb]=x;
+			if(*nb == nbb) return;
+
+		}
+		fp=fc;
+	}
+	*nb = nbb;
+}