OFFSET
0,2
COMMENTS
Generated by a sieve: start with natural numbers, remove those terms which occupy positions which are prime, leaving 1,4,6,8,9,10,12,14,15,16,18,...; remove those terms whose positions are primes plus one; leaving 1,4,9,12,15,16,18,...; remove those whose positions are primes plus two; continue. - Robert G. Wilson v, Jan 06 2008
Number of terms <= 10^k, k=0..: 1, 3, 8, 18, 34, 54, 80, 110, 147, 188, 235, 287, 345, 407, 475, ..., . - Robert G. Wilson v, Jan 06 2008
The first occurrence of a k-almost prime or 0 if not present: 1, 0, 4, 78, 16, 696, 5016, 95920, 46144, 10236034900, 600374208, 1613472, 21565696412400, 0, 24323590656, ..., . - Robert G. Wilson v, Jan 06 2008
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Chai Wah Wu, Table of n, a(n) for n = 0..900 (n = 0..503 from Robert G. Wilson v)
Popular Computing (Calabasas, CA), Contest 5 Results. Annotated and scanned copy of page PC41-15 of Vol. 4 (No. 41, Aug 1976).
MATHEMATICA
Composite[n_Integer] := FixedPoint[n + PrimePi@# + 1 &, n + PrimePi@n + 1]; NestList[ Composite@# &, 1, 45] (* Labos Elemer *)
With[{c=Select[Range[42000], CompositeQ]}, NestList[c[[#]]&, 1, 45]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 03 2018 *)
PROG
(Haskell)
a006508 n = a006508_list !! n
a006508_list = iterate a002808 1 -- Reinhard Zumkeller, Oct 24 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
Edited by Robert G. Wilson v, Jan 06 2008
STATUS
approved