src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Observable<Long> rangeLong(long start, long count) {
        if (count < 0) {
            throw new IllegalArgumentException("count >= 0 required but it was " + count);
        }

        if (count == 0) {
            return empty();
        }

        if (count == 1) {
            return just(start);
        }

        long end = start + (count - 1);
        if (start > 0 && end < 0) {
            throw new IllegalArgumentException("Overflow! start + count is bigger than Long.MAX_VALUE");
        }

        return RxJavaPlugins.onAssembly(new ObservableRangeLong(start, count));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Observable<Long> rangeLong(long start, long count) {
        if (count < 0) {
            throw new IllegalArgumentException("count >= 0 required but it was " + count);
        }

        if (count == 0) {
            return empty();
        }

        if (count == 1) {
            return just(start);
        }

        long end = start + (count - 1);
        if (start > 0 && end < 0) {
            throw new IllegalArgumentException("Overflow! start + count is bigger than Long.MAX_VALUE");
        }

        return RxJavaPlugins.onAssembly(new ObservableRangeLong(start, count));
    }
Returns an Observable that emits a sequence of Longs within a specified range.
<p>
<img width="640" height="195" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/rangeLong.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code rangeLong} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param start
the value of the first Long in the sequence
@param count
the number of sequential Longs to generate
@return an Observable that emits a range of sequential Longs
@throws IllegalArgumentException
if {@code count} is less than zero, or if {@code start} + {@code count} &minus; 1 exceeds
{@code Long.MAX_VALUE}
@see <a href="http://reactivex.io/documentation/operators/range.html">ReactiveX operators documentation: Range</a>