A255475(2^n-1).

Terms

a(0) =1a(1) =6a(2) =24a(3) =94a(4) =356a(5) =1338a(6) =5036a(7) =18942a(8) =71264a(9) =268114a(10) =1008728a(11) =3795162a(12) =14278632a(13) =53720914a(14) =202115688a(15) =760425734a(16) =2860971568a(17) =10763916406a(18) =40497394188a(19) =152364521390a(20) =573245456832