I have language
$L =\{ a^{3i} : i \in N_{0}\}$
if we choose sentence $a^{3p}$
We can decompose it such as
$x = a^{k}$ where k >= 0
$y = a^{l}$ where l >= 1
$z = a^{m}a^{3p-p}$ where m >= 0 ; k + l <= p ; k + l + m = p;
If we choose i = 2 we recieve
$a^{k}a^{2l}a^{m}a^{3p-p}$
which is $a^{3p+l}$ and we have to find that $3p + l \% 3 = 0$ but this is can be true only when p >= 3 , otherwise such case cannot occur , how to determine if this is regular or not?