(3*a(n)+1)/2^(2*n + 1) = 23-6*n.

Terms

a(0) =15a(1) =45a(2) =117a(3) =213a(4) =-171a(5) =-4779a(6) =-35499a(7) =-207531a(8) =-1092267a(9) =-5417643a(10) =-25864875a(11) =-120236715a(12) =-548055723a(13) =-2460658347a(14) =-10916375211a(15) =-47960468139a(16) =-209021741739a(17) =-904806443691