Zeros with multiplicity, Hasse derivatives and linear factors of general skew polynomials

In this work, multiplicities of zeros of skew polynomials are studied. Two distinct definitions are considered: First, $a$ is said to be a zero of $F$ of multiplicity $r$ if $(x-a)^r$ divides $F$ on the right; second, $a$ is said to be a zero of $F$ of multiplicity $r$ if some skew polynomial $P = (x-a_r) \cdots (x-a_2) (x-a_1)$, having $a_1 = a$ as its only right zero, divides $F$ on the right. Neither of these two notions implies the other for general skew polynomials. We show that, in the first case, Lam and Leroy's concept of P-independence does not behave naturally, whereas a union theorem still holds. In contrast, we show that P-independence behaves naturally for the second notion of multiplicities. As a consequence, we provide extensions of classical commutative results to general skew polynomials. These include: (1) The upper bound on the number of (P-independent) zeros (counting multiplicities) of a skew polynomial by its degree, and (2) The equivalence of P-independence, Hermite interpolation and the invertibility of confluent Vandermonde matrices (for which we introduce skew polynomial Hasse derivatives).