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Conditional list indexing


Hello everybody,
I'd like to know how I can get the indices of the elements in a list
that satisfy a certain condition. Here's an example: suppose I have
the list

a = Table[Prime[i],{i,10}];

that contains {2,3,5,7,11,13,17,19,23,29}.

If I execute the command

b = Select[a,#>15&];

b will contain {17,19,23,29}. I'd like to find a way to obtain a third
list, say c, that will contain {7,8,9,10}, i.e. the indices of the
elements in list a that are greater than 15.

Thank you,
Daniele



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