A183220 - OEIS

A183220

Complement of the octagonal numbers.

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

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..100.

FORMULA

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

EXAMPLE

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

MATHEMATICA

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

PROG