a(n) = 6*a(n-2) for n > 2; a(1) = 1, a(2) = 4.

Terms

a(0) =1a(1) =4a(2) =6a(3) =24a(4) =36a(5) =144a(6) =216a(7) =864a(8) =1296a(9) =5184a(10) =7776a(11) =31104a(12) =46656a(13) =186624a(14) =279936a(15) =1119744a(16) =1679616a(17) =6718464a(18) =10077696a(19) =40310784a(20) =60466176a(21) =241864704a(22) =362797056a(23) =1451188224a(24) =2176782336a(25) =8707129344a(26) =13060694016a(27) =52242776064a(28) =78364164096