A026238 a(n) = j if n is the j-th prime, else a(n) = k if n is the k-th composite.

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

Offset

2,2

Links

Reinhard Zumkeller, Table of n, a(n) for n = 2..10000

Formula

a(n) = A049084(n) + A066246(n) for n >= 2.

Mathematica

Table[If[PrimeQ[n], PrimePi[n], n-1-PrimePi[n]], {n, 2, 70}] (* Harvey P. Dale, Jun 06 2017 *)

Prog

(Haskell)
a026238 n = a049084 n + a066246 n  -- Reinhard Zumkeller, Jan 29 2014
(PARI) first(n)=my(c, p); vector(n-1, k, if(isprime(k+1), p++, c++)) \\ Charles R Greathouse IV, Sep 02 2015

Crossrefs

Cf. A049084, A066246, A026233, A239968.

Sequence in context: A344096 A253138 A283994 * A066136 A257902 A257909

Adjacent sequences: A026235 A026236 A026237 * A026239 A026240 A026241

Keyword

nonn

Author

Clark Kimberling

Status

approved