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Questions about objects such as functions, algorithms or data structures that are expressed using "smaller" instances of themselves.
1
vote
Recursive set for a string
How about
$$ v \in \Sigma \Longrightarrow v \in P $$
and $$v \in P \land e \in \Sigma \Longrightarrow eve \in P $$ so $$ eve \in P \land l \in \Sigma \Longrightarrow \boxed{level \in P}$$