%I #9 Sep 08 2022 08:44:40
%S 1,3,10,100,2800,229600,56022400,40896352000,89481218176000,
%T 587175753670912000,11557967535258231808000,
%U 682497982956998588262400000,120903152684866385913507635200000
%N a(0)=1, a(1)=3, a(n) = sum_{k=0}^{k=n-1} 3^k a(k).
%F a(n) = (3^(n-2) + 1) * a(n-1). - _Vincenzo Librandi_, Nov 10 2012
%o (Magma) I:=[1, 3, 10]; [n le 3 select I[n] else (3^(n-2)+1)*Self(n-1): n in [1..15]]; // _Vincenzo Librandi_, Nov 10 2012
%K nonn,easy
%O 0,2
%A _Olivier GĂ©rard_