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%I #8 Sep 08 2022 08:44:40
%S 1,7,50,2500,860000,2065720000,34720621760000,4084881150064000000,
%T 3364079361848306816000000,19393251433341842829490432000000,
%U 782587666186514855538867732678656000000
%N a(0)=1, a(1)=7, a(n) = sum_{k=0}^{k=n-1} 7^k a(k).
%F a(n) = (7^(n-2) + 1) * Self(n-1). - _Vincenzo Librandi_, Nov 11 2012
%o (Magma) I:=[1, 7, 50]; [n le 3 select I[n] else (7^(n-2)+1)*Self(n-1): n in [1..15]]; // _Vincenzo Librandi_, Nov 11 2012
%K nonn,easy
%O 0,2
%A _Olivier Gérard_