A123249 a(1) = 1. For n >= 2, a(n) = n + (smallest integer which is >= n and is missing from among the earlier terms of the sequence).

1, 4, 6, 9, 10, 13, 14, 16, 20, 21, 22, 24, 28, 29, 30, 33, 34, 36, 38, 43, 44, 45, 46, 49, 50, 52, 54, 59, 60, 61, 62, 64, 68, 69, 70, 73, 74, 77, 78, 80, 82, 84, 90, 91, 92, 93, 94, 96, 100, 101, 102, 105, 106, 109, 110, 112, 114, 116, 122, 123, 124, 125, 126, 129, 130

Offset

1,2

Links

G. C. Greubel, Table of n, a(n) for n = 1..1000

Example

5 is the smallest integer which does not occur among the first 3 terms of the sequence and which is >= 4. So a(4) = 4 + 5 = 9.

Mathematica

f[l_List] := Block[{n = Length[l] + 1, k = n}, While[MemberQ[l, k], k++ ]; Append[l, n + k]]; Nest[f, {1}, 70] (* Ray Chandler, Oct 09 2006 *)

Prog

(PARI) {m=65; w=vector(3*m); print1(a=1, ", "); for(n=2, m, k=n; while(w[k], k++); print1(a=n+k, ", "); w[a]=1)} \\ Klaus Brockhaus, Oct 09 2006

Crossrefs

Cf. A123088, A123721.

Sequence in context: A171401 A120329 A182800 * A171136 A189167 A283794

Adjacent sequences: A123246 A123247 A123248 * A123250 A123251 A123252

Keyword

easy,nonn

Author

Leroy Quet, Oct 08 2006

Extensions

Corrected and extended by Klaus Brockhaus and Ray Chandler, Oct 09 2006

Status

approved