chiburashka,
You might be able to cheat by having a look at the On-Line Encyclopedia of Integer Sequences. The problem with identifying the formula yourself is being able to constrain the possibilities. For instance does the next number depend on the previous number? To calculate the Nth number do all previous numbers up to N first? Is it linear or geometric?

To say it a different way - having a handful of numbers and being told to find the formula that generated those numbers without any additional information is nearly impossible.

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