A165955 n-th odd nonprime plus n-th even nonprime.

1, 13, 21, 29, 35, 39, 47, 51, 57, 65, 71, 75, 81, 85, 93, 97, 103, 111, 115, 121, 127, 131, 137, 141, 145, 151, 159, 167, 173, 177, 181, 185, 189, 193, 199, 205, 209, 217, 221, 225, 229, 237, 241, 247, 251, 257, 263, 267, 273, 277, 285, 289, 293, 297, 305, 313, 317, 321, 325

Offset: 1

Author

Juri-Stepan Gerasimov, Oct 01 2009

Keywords

• nonn

Examples

			a(1) = 1+0 = 1. a(2) = 9+4 = 13. a(3) = 15+6 = 21.
		

Links

• G. C. Greubel, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. A014076, A141468, A163300.

Programs

Mathematica

A014076 := Select[Range@500, ! PrimeQ@# && OddQ@# &]; A163300 := Drop[Range[0, 500, 2], {2}]; Table[(A163300[[n]] + A014076[[n]]), {n, 1, 50}] (* G. C. Greubel, Sep 17 2017 *)
Module[{nn=300,np,enp,onp,len},np=Select[Range[0,nn],!PrimeQ[#]&];enp= Select[ np,EvenQ];onp=Select[np,OddQ];len=Min[Length[enp], Length[ onp]]; Total/@Thread[{Take[enp,len],Take[onp,len]}]] (* Harvey P. Dale, Nov 28 2018 *)

Python

from sympy import primepi
def A165955(n):
    if n == 1: return 1
    m, k = n-1, primepi(n) + n - 1 + (n>>1)
    while m != k:
        m, k = k, primepi(k) + n - 1 + (k>>1)
    return m+(n<<1) # Chai Wah Wu, Aug 01 2024

Formula

a(n) = A014076(n) + A163300(n).

Extensions

Entries checked by R. J. Mathar, Oct 10 2009