a(n) = 87*2^n - 45.

Terms

a(0) =42a(1) =129a(2) =303a(3) =651a(4) =1347a(5) =2739a(6) =5523a(7) =11091a(8) =22227a(9) =44499a(10) =89043a(11) =178131a(12) =356307a(13) =712659a(14) =1425363a(15) =2850771a(16) =5701587a(17) =11403219a(18) =22806483a(19) =45613011a(20) =91226067a(21) =182452179a(22) =364904403a(23) =729808851a(24) =1459617747a(25) =2919235539a(26) =5838471123a(27) =11676942291a(28) =23353884627a(29) =46707769299