float *farray1();

float maxlag(a,b,n,dt,prec)
float *a,*b,dt,prec;
int n;
{
float *work1,*work2,save,max,len;
int m1,m2,i;

	m2=m1=min2n(n);
	len=m1*dt;
	while(dt>=prec)
	{ dt*=0.5;
	  m2*=2;
	}
	
	work1=farray1(0,m2+1);
	work2=farray1(0,m2+1);
	
	for(i=0;i<n;i++)
	{ work1[i]=a[i];
	  work2[i]=b[i];
	}
	
	for(i=n;i<m1;i++) work1[i]=work2[i]=0.0;
	
	cfftr(work1,m1);
	cfftr(work2,m1);
	
	for(i=0;i<1+m1/2;i++)
	{ save=work1[2*i];
	  work1[2*i]=save*work2[2*i]+work1[2*i+1]*work2[2*i+1];
	  work1[2*i+1]=work1[2*i+1]*work2[2*i]-save*work2[2*i+1];
	}
	free_farray1(work2,0);
	for(i=1+m1/2;i<1+m2/2;i++) work1[2*i]=work1[2*i+1]=0.0;
	
	cfftri(work1,m2);
	
	max=0.0;
	for(i=0;i<m2;i++) if(max<work1[i]) {max=work1[i];save=i;}
	save*=dt;
	if(save>0.5*len) save-=len;
	
	return(save);
	
}