Lowest positive almost central elements of U_q(sl⁽¹⁾(n|n)) (ngeq 2), U_q(sl⁽²⁾(2n|2n)) (ngeq 2) and U_q(sl⁽⁴⁾(2n+1|2n+1)) (ngeq 1) and their multi-parameter quantum affine superalgebras

Let $\pi:sl(n|n)\to A(n-1,n-1)$ be the natural epimorphism of Lie superalgebra. Then $\dim\ker\pi=1$. Let $\pi^{(t)}:sl^{(t)}(n|n)\to A^{(t)}(n-1,n-1)$ be the natural epimorphism, where $t=1,2,4$. Let $\{e_k|k\in{\mathbb{Z}}\}$ be the basis of $\ker\pi^{(t)}$ with $e_k\in sl^{(t)}(n|n)_{(a_tk+b_t)\delta}$, where $(a_1,b_1)=(1,0)$, $(a_2,b_2)=(2,-1)$ and $(a_4,b_4)=(4,-2)$. The main result of this paper is to explicitly describe an element of $U_q(sl^{(t)}(n|n))$ (and its multi-parameter version) corresponding to $e_1$ (i.e., $k=1$). As for $U_q(sl^{(1)}(n|n))$ (i.e., $t=1$), the author had already had explicit description for every $k$ in 1999.