A289256 - OEIS

%I #15 May 10 2021 02:36:12

%S 2,2,2,4,4,4,4,6,6,8,6,8,8,8,8,10,10,12,10,14,10,14,12,16,14,16,14,16,

%T 16,16,16,18,18,20,18,22,18,24,18,24,20,26,22,26,22,28,24,30,24,30,24,

%U 30,26,32,28,32,30,32,30,32,32,32,32,34,34,36,34,38,34,40,34,42,34,42,36,44,38,44,38,46,40,48

%N a(1) = a(2) = a(3) = 2;  a(n) = a(n-a(n-1)) + a(n-a(n-3)-1) for n > 3.

%H Altug Alkan, <a href="/A289256/a289256.png">Plot of a(n)-n/2 for n<=2^16</a>

%t a[n_] := a[n] = If[n <= 3, 2, a[n - a[n - 1]] + a[n - a[n - 3] - 1]]; Array[a, 82] (* _Michael De Vlieger_, Jun 30 2017 *)

%o (PARI) q=vector(10^5); q[1]=q[2]=q[3]=2;for(n=4, #q, q[n] = q[n-q[n-1]]+q[n-1-q[n-3]]); q

%o (Python)

%o l=[0, 2, 2, 2]

%o for n in range(4, 101): l+=[l[n - l[n - 1]] + l[n - l[n - 3] - 1], ]

%o print(l[1:]) # _Indranil Ghosh_, Jul 01 2017

%Y Cf. A004001, A005185.

%K nonn

%O 1,1

%A _Altug Alkan_, Jun 29 2017