A139636 If n = the k-th prime, then a(n) = the (k+1)th prime. If n = the k-th composite, then a(n) = the (k+1)th composite.

3, 5, 6, 7, 8, 11, 9, 10, 12, 13, 14, 17, 15, 16, 18, 19, 20, 23, 21, 22, 24, 29, 25, 26, 27, 28, 30, 31, 32, 37, 33, 34, 35, 36, 38, 41, 39, 40, 42, 43, 44, 47, 45, 46, 48, 53, 49, 50, 51, 52, 54, 59, 55, 56, 57, 58, 60, 61, 62, 67, 63, 64, 65, 66, 68, 71, 69, 70, 72, 73, 74

Offset

2,1

Comments

This is a permutation of the positive integers sans 1,2,4.

Links

Harvey P. Dale, Table of n, a(n) for n = 2..1000

Maple

A000040 := proc(n) ithprime(n) ; end: A002808 := proc(n) local a; if n = 1 then 4; else for a from A002808(n-1)+1 do if not isprime(a) then RETURN(a) ; fi ; od: fi ; end: A066246 := proc(n) local k ; if isprime(n) then 0 ; else for k from 1 do if A002808(k) = n then RETURN(k) ; fi ; od: fi ; end: A049084 := proc(n) if not isprime(n) then 0; else numtheory[pi](n) ; fi ; end: A139636 := proc(n) local k; if isprime(n) then k := A049084(n) ; RETURN(A000040(k+1)) ; else k := A066246(n) ; RETURN(A002808(k+1)) ; fi ; end: seq(A139636(n), n=2..160) ; # R. J. Mathar, May 12 2008

Mathematica

npc[n_]:=Module[{k=1}, If[PrimeQ[n], NextPrime[n], While[PrimeQ[n+k], k++]; n+k]]; Array[npc, 80, 2] (* Harvey P. Dale, Jun 26 2022 *)

Crossrefs

Cf. A139637.

Sequence in context: A079253 A076054 A253201 * A219922 A159559 A047583

Adjacent sequences: A139633 A139634 A139635 * A139637 A139638 A139639

Keyword

nonn

Author

Leroy Quet, Apr 28 2008

Extensions

More terms from R. J. Mathar, May 12 2008

Status

approved