Find the maximum difference in a sequence

Let us put another interview question solution with F# (functional way). The question is to find the x[a], x[b], where x[b] - x[a] -> maximum

we are going to find a tuple (a,b) where x[b] - x[a] -> maximum. Be careful b must be greater than a. So our first problem will be how to generate a sequence (a,b) where b>a and a between (0, L).

f(i) will generate a sequence from i to L.
f(i) -> [i..L] |> Seq.map (n-> (i,n))

[0..L]
|> Seq.map (i-> Seq.map (n->(i,n)) [i..L] ) //generate the (a,b) tuple
|> Seq.collect (n->n) //flatten the sequence
|> Seq.maxby ((a,b)->x[b]-x[a]) //find the max dif