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A220699 a(0)=0, a(1)=0; for n>1, a(n) = a(n-1) + (n-4)*a(n-2) + 1.

%I A220699 #19 Jun 02 2025 08:10:51
%S A220699 0,0,1,2,3,6,13,32,85,246,757,2480,8537,30858,116229,455668,1850417,
%T A220699 7774102,33679941,150291472,689170529,3244125554,15649195077,
%U A220699 77287580604,390271482145,2013310674830,10599283282021
%N A220699 a(0)=0, a(1)=0; for n>1, a(n) = a(n-1) + (n-4)*a(n-2) + 1.
%H A220699 Vincenzo Librandi, <a href="/A220699/b220699.txt">Table of n, a(n) for n = 0..800</a>
%F A220699 a(0)=a(1)=0, a(2)=1, a(n) = 2*a(n-1)+(n-5)*a(n-2)-(n-5)*a(n-3).
%F A220699 a(n) ~ (sqrt(Pi)/2+sqrt(2)) * n^(n/2-3/2)*exp(sqrt(n)-n/2-1/4) * (1-29/(24*sqrt(n))). - _Vaclav Kotesovec_, Dec 27 2012
%t A220699 RecurrenceTable[{a[1]==0, a[2]==0, a[n]==a[n-1] + (n-5) a[n-2] + 1}, a, {n, 40}]
%o A220699 (Magma) [n le 2 select 0 else Self(n-1)+(n-5)*Self(n-2) + 1: n in [1..30]];
%Y A220699 Cf. A185108, A185109, A185308, A185309, A186738, A186739, A213720, A187830.
%K A220699 nonn,easy
%O A220699 0,4
%A A220699 _Vincenzo Librandi_, Dec 25 2012