Shuffle algebra realizations for restricted Yangians

We study the shuffle algebra realization of the positive subalgebra $Y_n^{>}(\mathbb{k})$ of the Yangian associated to $\mathfrak{sl}_n$ over an algebraically closed field $\mathbb{k}$ of characteristic $p>2$. In contrast to the characteristic zero case, the natural homomorphism from $Y_n^{>}(\mathbb{k})$ to the modular shuffle algebra $W^{(n)}(\mathbb{k})$ is not an isomorphism. We determine its kernel and image, showing that the kernel is precisely the ideal generated by the $p$-center of $Y_n^{>}(\mathbb{k})$, while the image consists of elements satisfying an additional wheel condition related to the characteristic $p$, thus providing a shuffle algebra realization for the restricted Yangian $Y_n^{>,[p]}$. The proof relies on the specialization maps approach and the construction of the small Yangian $\bar{y}^{>}_n(\mathbb{k})$, obtained by the reduction modulo $p$ method from an integral form $\mathbf{Y}_n^>$ of the Yangian $Y_n^{>}$ associated to $\mathfrak{sl}_n$ over $\mathbb{C}$.