Poster we knocked up to illustrate mathematical concept (5,3)

I believe the answer is:

power set

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'up to illustrate mathematical concept' is the definition.
I can't judge whether this definition defines the answer.

'poster we knocked' is the wordplay.
'knocked' indicates an anagram.
'poster'+'we'='posterwe'
'posterwe' anagrammed gives 'POWER SET'.

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