Small sumsets in real line : a continuous 3k-4 theorem

We prove a continuous Freiman’s $3k-4$ theorem for small sumsets in $\mathbb{R}$ by using some ideas from Ruzsa’s work on measure of sumsets in $\mathbb{R}$ as well as some graphic representation of density functions of sets. We thereby get some structural properties of $A$, $B$ and $A+B$ when $\lambda (A+B)<\lambda (A)+\lambda (B)+\min (\lambda (A),\lambda (B))$. We also give some structural information for sets of large density with small sumset and characterize the extremal sets for which equality holds in the lower bounds for $\lambda (A+B)$.