Partial sums of round(4^n/7).

Terms

a(0) =0a(1) =1a(2) =3a(3) =12a(4) =49a(5) =195a(6) =780a(7) =3121a(8) =12483a(9) =49932a(10) =199729a(11) =798915a(12) =3195660a(13) =12782641a(14) =51130563a(15) =204522252a(16) =818089009a(17) =3272356035a(18) =13089424140a(19) =52357696561a(20) =209430786243a(21) =837723144972