OFFSET
1,1
COMMENTS
For small n's a(2n) >a(2n-1) (that is for small n's, n-th odd prime less than n-th odd composite number), while for large n's a(2n) <a(2n-1) (n-th odd prime larger than n-th odd composite number), cf. A129146
MATHEMATICA
Module[{nn=100, pr, cm, len}, pr=Prime[Range[2, nn+1]]; cm=Select[Range[ 9, 2nn+1, 2], CompositeQ]; len=Min[Length[pr], Length[cm]]; Riffle[Take[ pr, len], Take[cm, len]]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 10 2019 *)
PROG
(Python)
from sympy import prime, primepi
def A129145(n):
if n&1: return prime((n>>1)+2)
if n==2: return 9
r = n>>1
m, k = r, primepi(r) + r + (r>>1)
while m != k:
m, k = k, primepi(k) + r + (k>>1)
return m # Chai Wah Wu, Aug 01 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Apr 01 2007
STATUS
approved