# Recurrence Formulae for specified range

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Hi —
First time posting here. New to Quantrix. I couldn’t find the solution / answer to this either here or on the online user guide.
I am trying to write a recursion formula for a specific range within an item. Reason being is that I have to write the formula for revenue a couple different ways depending on the period. (i.e., 2015 – 2017 should be calculated as the forward year’s revenue divided by (1+growth rate) and 2019 and onwards should be calculated as previous year’s revenue multiplied by (1+growth rate). 2018 is a hardcoded value.
Below is my attempt
Period[this]:’2015′:Revenue.Total..period[this]:’2017′:Revenue.Total = Matrix2::period[NEXT]:revenue growth:’2015′..period[next]:revenue growth:’2017′
I get the error multiple references to items on category ‘Period.’ after ‘:’
Is there an easy way to accomplish this? Also, in general, an easy way to accomplish recursion in different parts of the same item?
Or would you, in general, split up the item into helper items and then consolidate? If so, that seems a bit painful compared to how you would accomplish in Excel.