On Freiman's 3k-4 theorem
One of the many theorems Freiman proved, in the second half of the twentieth century, in the subject which later came to be known as "structure theory of set addition", was 'Freiman's $3k-4$ theorem' for subsets of $\Z$. In this article we introduce concept of a new `structure' on finite subsets of integers. Sets with this structure are quite useful in additive number theory in some contexts. Also we give some criterion for subsets to posses this structure. Then this is used to establish an analog of Freiman's $3k-4$ theorem for the groups $\Z \times G,$ where $G$ is any abelian group.