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Selecting Element from List via Pattern Matching


 I am trying to extract from a Power Series all elements with a
certain exponent, e.g.
let us say I have generated a list via

t = Apply[List, Expand[(Sum[Subscript[a, i], {i, 1, 5}])^3]]

How can I then extract all terms with e.g. a_2^2 in it or any other

I have tried something like

Cases[t, Times[_, Power[Subscript[a,2],2]]]

but this somehow only works with the exponent n=1 and no others...

Does anybody know a solution to this pattern matching problem?


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