recursion is the ability to place one component inside another component, so is it only for the same kind such as NP with NP, PP with PP, and etc?


A recursive phrase structure rule is one with the same symbol occurring in the string on the right hand side of the arrow and on the left of the arrow. For instance "N -> N and N". A recursive set of phrase structure rules is one which permits a phrase structure derivation in which a symbol can be rewritten as a string containing that same symbol. For instance "NP -> Det N, Det -> NP 's", which permits the psg derivation:

Det N  
NP 's N  

whose last line has "NP" recurring in the string "NP 's N".

In order to generate an infinite number of sentences, a psg has to be a recursive set of rules.

A tree which has a symbol dominating itself could only be generated by a recursive psg. However, trees are often cited which are supposed to have some source other than a psg, so just because you see a tree with a recursive symbol, that doesn't necessarily have any interesting grammatical consequences.

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