A073898 a(1) = 1; for n>1, a(n) = smallest even or odd number not occurring earlier accordingly as n is prime or composite.

1, 2, 4, 3, 6, 5, 8, 7, 9, 11, 10, 13, 12, 15, 17, 19, 14, 21, 16, 23, 25, 27, 18, 29, 31, 33, 35, 37, 20, 39, 22, 41, 43, 45, 47, 49, 24, 51, 53, 55, 26, 57, 28, 59, 61, 63, 30, 65, 67, 69, 71, 73, 32, 75, 77, 79, 81, 83, 34, 85, 36, 87, 89, 91, 93, 95, 38, 97, 99, 101, 40, 103

Offset

1,2

Comments

Inverse of A073846. - Chayim Lowen, Oct 28 2015

Links

Table of n, a(n) for n=1..72.

Formula

a(n) = (4*pi(n)-2*n-1)*(pi(n)-pi(n-1)) + 2*(n - pi(n-1)) - 1, where pi = A000720. - Robert Israel, Oct 28 2015

Maple

A073898 :=proc(nmax) local a, n, k; a := [1] ; while nops(a) < nmax do n := nops(a)+1 ; if isprime(n) then k :=2; else k :=1; fi ; while k in a do k := k+2 ; od ; a := [op(a), k] ; od ; RETURN(a) ; end: op(A073898(80)) ; # R. J. Mathar, Jun 27 2007

Mathematica

Table[(4 PrimePi[n] - 2 n - 1)*(PrimePi[n] - PrimePi[n - 1]) + 2 (n - PrimePi[n - 1]) - 1, {n, 72}] (* Michael De Vlieger, Nov 11 2015 *)

Prog

(PARI) vector(100, n, (4*primepi(n)-2*n-1)*(primepi(n)-primepi(n-1))+2*(n-primepi(n-1))-1) \\ Altug Alkan, Oct 29 2015

Crossrefs

Cf. A000720, A073897.

Cf. A073846.

Sequence in context: A135761 A086305 A328047 * A067747 A073846 A110458

Adjacent sequences: A073895 A073896 A073897 * A073899 A073900 A073901

Keyword

easy,nonn

Author

Amarnath Murthy, Aug 18 2002

Extensions

Corrected and extended by R. J. Mathar, Jun 27 2007

Status

approved