I tried a negative series and a decimal series, just in case the rule was increasing natural numbers. I tried very large numbers to see if there was a limit to the rule, and a series that had large contrast in between each element. For fun I tried to see if the app would recognize "pi", "i", or "e", but, perhaps unsurprisingly, it did not.
I would have checked complex numbers for fun but there is no reasonable ordering of the complex numbers in the way there is for real numbers. It's too bad it didn't recognize pi or e.
