This should work:

source.retryWhen { errors ->  
  errors
    .zipWith(Observable.range(1, 3)) { n, i -> i }
    .flatMap { retryCount -> Observable.timer(Math.pow(5.0, retryCount.toDouble()).toLong(), TimeUnit.SECONDS) }
}

I suppose you're on RxJava1?

Now you beat me to it xD

.retryWhen { it -> it.zipWith(Observable.range(1,3), { n, i -> i }) .flatMap { retryCount -> Observable.timer( Math.pow(5.toDouble(), retryCount.toDouble()).toLong(), TimeUnit.SECONDS) } }

Haha. Yea. Good catch @robin. I didn't attempt to see it compile 🙈