Textbook Problem

The function that associates a language to each regular expression over an alphabet $\sum$ is defined recursively. The base for the definition is the statement that $L(\varnothing )=$_______ , $L\left(\lambda \right)=$_______ , and $L\left(a\right)=$_______ for every a in $\sum$ . The recursion for the definition specifies that if $L\left(r\right)$ and $L\left(r\text{'}\right)$ are the languages defined by the regular expressions r and r’ over $\sum$ , then $L\left(rr\text{'}\right)=$_______ , $L\left(r|r\text{'}\right)=$_______ , and $L\left(r*\right)=$_______ .