# Expanding sets in cyclic group of prime order.

Let $S_1,S_2,\ldots S_k$ be a sequence of sets. We will call this sequence expanding if $S_i$ is not covered by $S_1,\ldots S_{i-1}$ i.e. $S_i$ contains at least one new element. Let $C_p$ be a cyclic group of size prime order $p$.

It is easy to show that for every $A\subset C_p$ there exists at least $k=\frac{p}{|A|}$ elements $a_1,\ldots a_k$, such that the sets $A+a_i$ are expanding.(Since each time we cover at most |A| new elements and we can cover all elements)

My question is if it is possible to improve this bound for $A$ of size at most $p/2$ to $k=\frac{p\log|A|}{|A|}$?

• Here is important that group is of prime order. Else if A is $pC_m$, where p divisor of $m$ than $k=\frac{m}{|A|}$ – Klim Efremenko Jan 30 '12 at 16:13
• Let $a_0,a_1,a_2,\dots,a_k$ be a list of elements from $C_p$, $A \subset C_p$ and $A_i=A+a_i.$ Are you looking to have $A_0,A_0\cup A_1 ,A_0\cup A_1 \cup A_2 \dots?$ an expanding sequence? – Aaron Meyerowitz Jan 30 '12 at 18:25
• By the definition $A_0,A_1,\ldots$ expanding if and only iff $A_0,A_0\cup A_1,A_0\cup A_1\cup A_2,…$ is expanding. I am looking that $A_1,A_2,\ldots$ is expanding. Where $A_i=A+a_i$ – Klim Efremenko Jan 30 '12 at 19:03