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a(0)=1, a(1)=2, a(n) = sum_{k=0}^{k=n-1} 2^k a(k).
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%I #11 Sep 08 2022 08:44:40

%S 1,2,5,25,225,3825,126225,8204625,1058396625,272007932625,

%T 139540069436625,143028571172540625,293065542332535740625,

%U 1200689526936398929340625,9837249294189916428087740625,161183329685301780674217630140625

%N a(0)=1, a(1)=2, a(n) = sum_{k=0}^{k=n-1} 2^k a(k).

%H Vincenzo Librandi, <a href="/A015486/b015486.txt">Table of n, a(n) for n = 0..70</a>

%o (Magma) I:=[1, 2, 5]; [n le 3 select I[n] else (2^(n-2)+1)*Self(n-1): n in [1..18]]; // _Vincenzo Librandi_, Nov 10 2012

%K nonn,easy

%O 0,2

%A _Olivier GĂ©rard_