a(n) = (13*3^n-1)/2.

Terms

a(0) =6a(1) =19a(2) =58a(3) =175a(4) =526a(5) =1579a(6) =4738a(7) =14215a(8) =42646a(9) =127939a(10) =383818a(11) =1151455a(12) =3454366a(13) =10363099a(14) =31089298a(15) =93267895a(16) =279803686a(17) =839411059a(18) =2518233178a(19) =7554699535a(20) =22664098606a(21) =67992295819a(22) =203976887458a(23) =611930662375