We know that Cox-de Boor recursion formula can be used to compute the B-spline basis function.
$ N_l^n(u)=\frac{u-u_{l-1}}{u_{l+n-1}-u_{l-1}}N^{n-1}_l(u)+ \frac{u_{l+n}-u}{u_{l+n}-u_{l}}N^{n-1}_{l+1}(u) $ where $u_i$ are given knots and $N_l^n(u)$ is the $l$th B-spine basis of degree n.
I have read books about it (eg. Curves and Surfaces for CAGD: A Practical Guide ). And I have also done some search on the Internet. But none of them mention the using condition of the formula.
I doubt this formula doesn't work when there are multiple knots since in this case the denominator $u_{l+n}-u_l$ can be zero.
Can you please help me? Thank you.