a(0)=0; a(1)=1; a(n) = triangular number at index 5*2^(n-2)-1.

Terms

a(0) =0a(1) =1a(2) =10a(3) =45a(4) =190a(5) =780a(6) =3160a(7) =12720a(8) =51040a(9) =204480a(10) =818560a(11) =3275520a(12) =13104640a(13) =52423680a(14) =209704960a(15) =838840320a(16) =3355402240a(17) =13421690880a(18) =53686927360a(19) =214748037120a(20) =858992803840