Avoiding sorting for min/max aggregate functions

Is it possible to avoid sorting (SORT AGGREGATE) in the following case:

explain plan for
Select max(a) from t
where sign(b-500)=1 and sign(c-7)=1
/

select * from table(dbms_xplan.display());

All columns are not null and numbers. Functional index is created and analyzed.

The statement

explain plan for
Select * from (

    Select a from t

        where sign(b-500)=1 and sign(c-7)=1     Order by

        sign(b-500),sign(c-7), a desc
) where rownum<=1
/

select * from table(dbms_xplan.display());

works fine (just an index scan(INDEX RANGE SCAN), no sorting operations)

Test DDL:

create table
t
(a number not null, b number not null, c number not null );

insert into t select mod(object_id+1245,1001), mod(object_id+4545,1111), mod(object_id+774545,13) from all_objects where rownum<=10000; commit;

Create index TESTIDX on T
(sign(b-500)
,sign(c-7)
,a DESC)
compute statistics
/

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