src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapObservable(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new SingleFlatMapObservable<T, R>(this, mapper));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapObservable(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new SingleFlatMapObservable<T, R>(this, mapper));
    }
Returns an Observable that is based on applying a specified function to the item emitted by the source Single,
where that function returns an ObservableSource.
<p>
<img width="640" height="300" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.flatMapObservable.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code flatMapObservable} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <R> the result value type
@param mapper
a function that, when applied to the item emitted by the source Single, returns an ObservableSource
@return the Observable returned from {@code func} when applied to the item emitted by the source Single
@see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>