a(1) = 1; a(n) = Sum_{k=1..n-1} phi(a(k)*a(n-k)), where phi(m) is the totient function.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =6a(5) =10a(6) =16a(7) =38a(8) =80a(9) =160a(10) =328a(11) =752a(12) =1712a(13) =4000a(14) =8736a(15) =16940a(16) =34816a(17) =83904a(18) =178304a(19) =433408a(20) =1031552a(21) =2601600a(22) =5193984a(23) =11318272a(24) =26631680a(25) =62674688a(26) =160582656a(27) =336680960a(28) =715578368a(29) =1829193728