Initial data commit

Task/Van-Eck-sequence/F-Sharp/van-eck-sequence-4.fs

@@ -0,0 +1,21 @@
+open System.Collections.Generic
+let VanEck() =
+    let rec _vanEck (num:int) (pos:int) (lastOccurence:Dictionary<int, int>) =
+        match lastOccurence.TryGetValue num with
+        | (true, position) ->
+            set num pos (pos - position) lastOccurence
+        | _ ->
+            set num pos 0 lastOccurence
+    and set num pos next lastOccurenceByNumber = seq {
+            lastOccurenceByNumber.[num] <- pos
+            yield next
+            yield! _vanEck next (pos + 1) lastOccurenceByNumber
+        }
+
+    seq {
+        yield 0
+        yield! _vanEck 0 1 (new Dictionary<int, int>())
+    }
+
+VanEck() |> Seq.take 10 |> Seq.map (sprintf "%i") |> String.concat " " |> printfn "The first ten terms of the sequence : %s"
+VanEck() |> Seq.skip 990 |> Seq.take 10 |> Seq.map (sprintf "%i") |> String.concat " " |> printfn "Terms 991 - to - 1000 of the sequence : %s"