Remarks on the sequential effect algebras

In this paper, first, we answer affirmatively an open problem which was presented in 2005 by professor Gudder on the sub-sequential effect algebras. That is, we prove that if $(E,0,1, \oplus, \circ)$ is a sequential effect algebra and $A$ is a commutative subset of $E$, then the sub-sequential effect algebra $\bar{A}$ generated by $A$ is also commutative. Next, we also study the following uniqueness problem: If $na=nb=c$ for some positive integer $n\geq 2$, then under what conditions $a=b$ hold? We prove that if $c$ is a sharp element of $E$ and $a|b$, then $a=b$. We give also two examples to show that neither of the above two conditions can be discarded.