%I #11 Oct 05 2024 10:57:27

%S 2,3,4,5,6,7,9,10,11,12,13,14,15,16,17,18,19,20,22,23,24,25,26,27,28,

%T 29,30,31,32,33,34,35,36,37,38,39,41,42,43,44,45,46,47,48,49,50,51,52,

%U 53,54,55,56,57,58,59,60,61,62,63,64,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,97,98,99,100,101,102,103,104,105,106

%N Complement of the octagonal numbers.

%F a(n) = n + floor(1/2+(n/3)^(1/2)).

%e Octagonal numbers: (1,8,21,40,65,...), so that we get (2,3,4,5,6,7,9,10,...,20,22,...,39,41...).

%t Table[n+Floor[1/2+(n/3)^(1/2)], {n,100}]

%o (Python)

%o from math import isqrt

%o def A183220(n): return n+(isqrt((n<<2)//3)+1>>1) # _Chai Wah Wu_, Oct 05 2024

%Y Cf. A000567 (octagonal numbers).

%K nonn

%O 1,1

%A _Clark Kimberling_, Jan 01 2011