The Fibonacci sequence is not much of a "formula", it's simply adding the previous two values in the sequence together. If you don't know that, the interviewer

The Fibonacci sequence is not much of a "formula", it's simply adding the previous two values in the sequence together. If you don't know that, the interviewer would probably happily tell you what it is, because the question probably aims at how you would actually implement it. Though not knowing what Fibonacci is might still be a little bit suspect, given how often it is used in examples and literature in general as a "very simple" recursive function. One might think that the candidate hasn't explored their field very much.