a(n) = 164*2^n - 140.

Terms

a(0) =24a(1) =188a(2) =516a(3) =1172a(4) =2484a(5) =5108a(6) =10356a(7) =20852a(8) =41844a(9) =83828a(10) =167796a(11) =335732a(12) =671604a(13) =1343348a(14) =2686836a(15) =5373812a(16) =10747764a(17) =21495668a(18) =42991476a(19) =85983092a(20) =171966324a(21) =343932788a(22) =687865716a(23) =1375731572a(24) =2751463284a(25) =5502926708a(26) =11005853556a(27) =22011707252a(28) =44023414644a(29) =88046829428