  One of the classic results of group theory is the so-called Schur theorem. It
states that if the central factor-group $G/\zeta(G)$ of a group $G$ is finite,
then its derived subgroup $[G,G]$ is also finite. This result has numerous
generalizations and modifications in group theory. At the same time, similar
investigations were conducted in other algebraic structures. In 2016, L.A.
Kurdachenko, J. Otal and O.O. Pypka proved an analogue of Schur theorem for
Leibniz algebras: if central factor-algebra $L/\zeta(L)$ of Leibniz algebra $L$
has finite dimension, then its derived ideal $[L,L]$ is also
finite-dimensional. Moreover, they also proved a slightly modified analogue of
Schur theorem: if the codimensions of the left $\zeta^{l}(L)$ and right
$\zeta^{r}(L)$ centers of Leibniz algebra $L$ are finite, then its derived
ideal $[L,L]$ is also finite-dimensional. One of the generalizations of Leibniz
algebras is the so-called Leibniz $n$-algebras. Therefore, the question of
proving analogs of the above results for this type of algebras naturally
arises. In this article, we prove the analogues of the two mentioned theorems
for Leibniz 3-algebras.
