On self-duality and hulls of cyclic codes over $frac{mathbb{F}_{2^m}[u]}{langle u^krangle}$ with oddly even length

Let $mathbb{F}_{2^m}$ be a finite field of $2^m$ elements, and $R=mathbb{F}_{2^m}[u]/langle u^krangle=mathbb{F}_{2^m}+umathbb{F}_{2^m}+ldots+u^{k-1}mathbb{F}_{2^m}$ ($u^k=0$) where $k$ is an integer satisfying $kgeq 2$. For any odd positive integer $n$, an explicit representation for every self-dual cyclic code over $R$ of length $2n$ and a mass formula to count the number of these codes are given first. Then a generator matrix is provided for the self-dual and $2$-quasi-cyclic code of length $4n$ over $mathbb{F}_{2^m}$ derived by every self-dual cyclic code of length $2n$ over $mathbb{F}_{2^m}+umathbb{F}_{2^m}$ and a Gray map from $mathbb{F}_{2^m}+umathbb{F}_{2^m}$ onto $mathbb{F}_{2^m}^2$. Finally, the hull of each cyclic code with length $2n$ over $mathbb{F}_{2^m}+umathbb{F}_{2^m}$ is determined and all distinct self-orthogonal cyclic codes of length $2n$ over $mathbb{F}_{2^m}+umathbb{F}_{2^m}$ are listed.