A139637 - OEIS

A139637

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. a(2) = 1. a(4) = 0.

1, 2, 0, 3, 4, 5, 6, 8, 9, 7, 10, 11, 12, 14, 15, 13, 16, 17, 18, 20, 21, 19, 22, 24, 25, 26, 27, 23, 28, 29, 30, 32, 33, 34, 35, 31, 36, 38, 39, 37, 40, 41, 42, 44, 45, 43, 46, 48, 49, 50, 51, 47, 52, 54, 55, 56, 57, 53, 58, 59, 60, 62, 63, 64, 65, 61, 66, 68, 69, 67, 70, 71, 72

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OFFSET

2,2

COMMENTS

This is a permutation of the nonnegative integers.

LINKS

Table of n, a(n) for n=2..74.

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: A139637 := proc(n) local k; if n = 2 then 1; elif n = 4 then 0 ; else if isprime(n) then k := A049084(n) ; A000040(k-1) ; else k := A066246(n) ; A002808(k-1) ; fi ; fi ; end: seq(A139637(n), n=2..160) ; # R. J. Mathar, May 12 2008

CROSSREFS

Cf. A139636.

Sequence in context: A198370 A173517 A109921 * A307734 A363346 A243202

Adjacent sequences: A139634 A139635 A139636 * A139638 A139639 A139640

KEYWORD

nonn

AUTHOR

Leroy Quet, Apr 28 2008

EXTENSIONS

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

STATUS

approved