"Truly random" is almost meaningless: for any finite sequence of numbers there are always multiple options for what the next number might be. You can always adjust your 'formula' to account for whatever other number or sequence of numbers gets thrown out, so given a finite sequence you can never be completely sure that you have the 'right' formula. And for an infinite sequence, well then you already have the whole thing, so there's nothing to predict.
The only meaningful definition of random is *statistically* random, meaning "given the start of a sequence, do we have a greater than even chance of predicting some parts of the sequence we haven't seen yet?" And people have repeatedly been shown to be very bad at that. This arguing of "you can't predict my next number so it's random" is irrelevant: you can *never* predict the next number with 100% accuracy given a prior sequence. Never.
1,2,4,8,16,?
32? If they're powers of 2.
31? if they're the maximum number of pieces you get from connecting n points on a circle into a complete graph.
Any 5 points determine a quartic, so you could setup ax^4+bx^3+cx^2+dx+e and sub in the points (0,1), (1,2), (2,4), (3,8), (4,16) into that equation and solve for the coefficients, then take the value at x=5 as your next number. Or you could create any sequence of x values (say squares), and plug in the next x value in that sequence.
If you're using random to mean anything other than statistics, you're doing it wrong. 100% predictability of a sequence when you don't know what generates it is a useless thing to even bother talking about, because it never exists, even mathematically.