a(n) = Sum_{d|6} phi(d)*n^(6/d).

Terms

a(0) =0a(1) =6a(2) =84a(3) =780a(4) =4200a(5) =15810a(6) =46956a(7) =118104a(8) =262800a(9) =532350a(10) =1001220a(11) =1773156a(12) =2988024a(13) =4829370a(14) =7532700a(15) =11394480a(16) =16781856a(17) =24143094a(18) =34018740a(19) =47053500a(20) =64008840a(21) =85776306a(22) =113391564a(23) =148049160a(24) =191118000a(25) =244157550a(26) =308934756