a(n) = 172*2^n - 176.

Terms

a(0) =-4a(1) =168a(2) =512a(3) =1200a(4) =2576a(5) =5328a(6) =10832a(7) =21840a(8) =43856a(9) =87888a(10) =175952a(11) =352080a(12) =704336a(13) =1408848a(14) =2817872a(15) =5635920a(16) =11272016a(17) =22544208a(18) =45088592a(19) =90177360a(20) =180354896a(21) =360709968a(22) =721420112a(23) =1442840400a(24) =2885680976a(25) =5771362128a(26) =11542724432a(27) =23085449040a(28) =46170898256a(29) =92341796688