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Final Friday Fix: November Challenge

25 November 2016

Recursion Excursion

Sometimes, future results are based on what has happened previously. For instance, creating a sequence which is based on previous values in that sequence, e.g. 10, 9, 8, 7, … This sequence subtracts one from the previous number. Some sequences are surprisingly awkward to forecast. This month’s challenge is one such infamous example…

The Challenge

Feel free to download the attached Excel file:

In cells F13:F37 of the above example, I have created what is known as a recursive sequence, i.e. a sequence that is based on earlier values in the same sequence.  In this instance, the numbers are calculated by adding the previous two numbers together. 

The question is: can you write a formula to replace the current calculation in cell I19 that will predict the seventh value (pictured, see cells H19:I19 in the above image), the 15th, the 29th, 107th..?

Just one restriction: the formula may not refer to the data table or any extension / variant of it.


Sound easy?  Try it.  We’ll publish our suggested solution in Monday’s blog.  Have a great weekend!