for( my $k = 0 ; $k < $n ; $k++ ) {
        next if $k == $j;
        my $generic_pol = Math::Polynomial->new( 1, -$pair_points[$k]->[0] )
                        / ( $pair_points[$j]->[0] - $pair_points[$k]->[0]);
        $pol *= $generic_pol;
    }