Program.cs

class Program
    {
        static void Main(string[] args)
        {
            Func<int, int, int> add = (a, b) => a + b;

            var obs1 = Observable.Empty<int>().Scan(add);
            var obs2 = Observable.Empty<int>().Scan(0, add);

            var obs3 = Observable.Range(1, 3).Scan(add);
            var obs4 = Observable.Range(1, 3).Scan(0, add);

            obs1.Subscribe(CreateObserver<int>("1"));
            obs2.Subscribe(CreateObserver<int>("2"));

            obs3.Subscribe(CreateObserver<int>("3"));
            obs4.Subscribe(CreateObserver<int>("4"));

            Console.ReadLine();
        }

        public static IObserver<T> CreateObserver<T>(string name)
        {
            return Observer.Create<T>(
                next =>
                {
                    Console.WriteLine("next {0}:{1}", name, next);
                },
                error =>
                {
                    Console.WriteLine(error);
                },
                () =>
                {
                    Console.WriteLine("completed {0}", name);
                }
            );
        }
    }

results.txt

completed 1
completed 2
next 3:1
next 3:3
next 3:6
completed 3
next 4:1
next 4:3
next 4:6
completed 4