a(n) = phi(3^n-1), where phi is Euler's totient function (A000010).

Terms

a(0) =1a(1) =4a(2) =12a(3) =32a(4) =110a(5) =288a(6) =1092a(7) =2560a(8) =9072a(9) =26400a(10) =84700a(11) =165888a(12) =797160a(13) =2384928a(14) =6019200a(15) =15728640a(16) =64533700a(17) =141087744a(18) =580765248a(19) =1246080000a(20) =4823425152a(21) =14758128000a(22) =46070066188a(23) =85996339200a(24) =385087175000