sub lentzCF {
    # continued fraction using lentz method Numerical Recipes
    # p. 171. Arguments are as, bs, maximum number of iterations and
    # smallness parameter
    # a0+b1/a1+b2+...
    my $as=shift;
    my $bs=shift;
    my $max=shift;
    my $small=shift;
    my $tiny=r2C(1.e-30);
    my $converged=0;
    my $fn=$as->slice(0);
    $fn=$tiny if all($fn==0);
    my $n=1;
    my ($fnm1, $Cnm1, $Dnm1)=($fn, $fn, r2C(0)); #previous coeffs.
    my ($Cn, $Dn); #current coeffs.
    my $Deltan;
    while($n<$max){
	$Dn=$as->slice($n)+$bs->slice($n)*$Dnm1;
	$Dn=$tiny if all($Dn==0);
	$Cn=$as->slice($n)+$bs->slice($n)/$Cnm1;
	$Cn=$tiny if all($Cn==0);
	$Dn=1/$Dn;
	$Deltan=$Cn*$Dn;
	$fn=$fnm1*$Deltan;
	last if $converged=$Deltan->approx(1, $small)->all;
	$fnm1=$fn;
	$Dnm1=$Dn;
	$Cnm1=$Cn;
	$n++;
    }
    $fn = $fn->slice("(0)");
    return wantarray? ($fn, $n): $fn;
}