A026233 a(n) = j if n is the j-th prime, else a(n) = k if n is the k-th nonprime.

1, 1, 2, 2, 3, 3, 4, 4, 5, 6, 5, 7, 6, 8, 9, 10, 7, 11, 8, 12, 13, 14, 9, 15, 16, 17, 18, 19, 10, 20, 11, 21, 22, 23, 24, 25, 12, 26, 27, 28, 13, 29, 14, 30, 31, 32, 15, 33, 34, 35, 36, 37, 16, 38, 39, 40, 41, 42, 17, 43, 18, 44, 45, 46, 47, 48

Offset

1,3

Comments

Each n occurs at two positions, the distances between them are: 1, 1, 1, 1, 2, 3, 5, 5, 8, 13, 13, 17, 20, 21, 23, 28, 33, 34, 39, 41, 41, ... - Zak Seidov, Mar 06 2011

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Formula

a(n) = A049084(n) + A066246(n) + A000007(A010051(n)). - Reinhard Zumkeller, Feb 12 2014

a(n) = A049084(n) + A239968(n). - Reinhard Zumkeller, Mar 30 2014

Mathematica

m=100; pr=Prime[Range[m]]; npr=Select[Range[m], !PrimeQ[#]&];
a[n_]:=If[PrimeQ[n], PrimePi[n], Position[npr, n][[1, 1]]]; Table[a[n], {n, m}] (* Zak Seidov Mar 05 2011 *) s=Range[500]; Do[s=Insert[s, n, Prime[n]], {n, 100}]; s (* Zak Seidov, Mar 05 2011 *)

Prog

(Haskell)
a026233 n = a049084 n + a239968 n
-- Reinhard Zumkeller, Mar 30 2014, Feb 12 2014
(PARI) first(n)=my(p, c); vector(n, k, if(isprime(k), p++, c++)) \\ Charles R Greathouse IV, Sep 02 2015

Crossrefs

Cf. A026238.

Sequence in context: A256991 A267110 A026261 * A222422 A194204 A348110

Adjacent sequences: A026230 A026231 A026232 * A026234 A026235 A026236

Keyword

nonn

Author

Clark Kimberling

Status

approved