I remember reading this problem in some contest, but I don't remember where. I think the contest was 2016 or so. I searched on a few websites with no luck. Do you happen to know what contest it was?

The problem is: Given integers *a*_{1}, ..., *a*_{n}, We define *x*_{0} = 1 and for every . Find *x*_{n}. The solution was *O*(*n* * *polylog*(*n*)) with FFT.