The sequence a[n]+b[n]+c[n]+d[n] defined in A126940.

Terms

a(0) =1a(1) =5a(2) =15a(3) =47a(4) =151a(5) =479a(6) =1519a(7) =4827a(8) =15333a(9) =48693a(10) =154651a(11) =491189a(12) =1560039a(13) =4954755a(14) =15736581a(15) =49980243a(16) =158739915a(17) =504166505a(18) =1601260009a(19) =5085687979a(20) =16152418715a(21) =51300951395a(22) =162934583329a(23) =517489007515