OFFSET
1,2
COMMENTS
This is to A060000 as tribonacci A000073 is to Fibonacci A000045. Let H be the set of positive numbers less than a(n) which are not equal to some a(i), i < n. This H is the 'set of holes so far'. If H is nonempty, then define a(n+1) = minimum(H). Otherwise define a(n+1) = a(n-2) + a(n-1) + a(n). Permutation of the natural numbers with inverse not yet in the OEIS.
LINKS
Ivan Neretin, Table of n, a(n) for n = 1..10000
MATHEMATICA
a[n_] := a[n] = If[n < 4, n, Block[{s = Array[a, n-1]}, s = Complement[ Range[ Max@s], s]; If[s == {}, a[n - 1] + a[n - 2] + a[n - 3], First[s]]]]; Array[a, 80] (* Giovanni Resta, Jun 20 2016 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, May 04 2008
EXTENSIONS
Data corrected by Giovanni Resta, Jun 20 2016
STATUS
approved