Where 2^n appears in A340488 for the first time.

Terms

a(0) =3a(1) =6a(2) =16a(3) =56a(4) =216a(5) =856a(6) =3416a(7) =13656a(8) =54616a(9) =218456a(10) =873816a(11) =3495256a(12) =13981016a(13) =55924056a(14) =223696216a(15) =894784856a(16) =3579139416a(17) =14316557656a(18) =57266230616a(19) =229064922456a(20) =916259689816