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a(n) = c(n-1) + c(n-3) where c is the sequence of numbers not in a.
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%I #6 Mar 29 2017 20:28:41

%S 1,2,4,9,12,14,17,19,23,26,29,33,36,39,42,45,47,51,53,57,59,62,65,67,

%T 71,73,77,79,83,85,89,92,95,98,101,104,107,110,113,116,119,123,125,

%U 129,132,135

%N a(n) = c(n-1) + c(n-3) where c is the sequence of numbers not in a.

%H Ivan Neretin, <a href="/A022428/b022428.txt">Table of n, a(n) for n = 0..10000</a>

%t Fold[Append[#1, Plus @@ Complement[Range[Max@#1 + 3], #1][[{#2, #2 + 2}]]] &, {1, 2, 4}, Range[43]] (* _Ivan Neretin_, Mar 29 2017 *)

%Y Cf. A022424 and references therein.

%K nonn

%O 0,2

%A _Clark Kimberling_