A336407 a(n) is the number of composites < n-th odd composite.

3, 7, 11, 14, 16, 20, 22, 25, 29, 32, 34, 37, 39, 43, 45, 48, 52, 54, 57, 60, 62, 65, 67, 69, 72, 76, 80, 83, 85, 87, 89, 91, 93, 96, 99, 101, 105, 107, 109, 111, 115, 117, 120, 122, 125, 128, 130, 133, 135, 139, 141, 143, 145, 149, 153, 155, 157, 159, 161

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

a(2) is the number of the numbers 4, 6, 8, 9, 10, 12, 14, these being the composites that are less than 15, which is the 2nd odd composite.

Maple

Cons:= remove(isprime, [seq(i, i=4..10^3)]):
select(i -> Cons[i+1]::odd, [$1..nops(Cons)-1]); # Robert Israel, Jun 07 2026

Mathematica

z = 400; p = Prime[Range[z]];
c = Select[Range[2, z], ! PrimeQ@# &];  (* A002808 *)
d = Select[Range[2, z], ! PrimeQ@# && OddQ@# &];  (* A014076 *)
f[n_] := Select[c, # < d[[n]] &];
g[n_] := d[[n]] + Select[c, # < d[[n]] &];
q[n_] := Length[Intersection[p, g[n]]];
tq = Table[q[n], {n, 1, 120}] (* A336406 *)
tc = Table[Length[f[n]], {n, 1, 120}] (* A336407 *)
m = Min[Length[tq], Length[tc]]; Take[tc, m] - Take[tq, m] (* A336408 *)

Prog

(PARI) n=0; forcomposite(x=4, 210, if(x%2, print1(n, ", ")); n++) \\ Hugo Pfoertner, Jul 26 2020
(Python)
from sympy import primepi
def A336407(n):
    if n == 1: return 3
    m, k = n, (r:=primepi(n)) + n + (n>>1)
    while m != k:
        m, k = k, (r:=primepi(k)) + n + (k>>1)
    return m-r-2 # Chai Wah Wu, Jul 31 2024

Crossrefs

Cf. A000040, A002808, A014076, A336406, A336408.

Sequence in context: A354904 A277009 A277019 * A310201 A310202 A310203

Adjacent sequences: A336404 A336405 A336406 * A336408 A336409 A336410

Keyword

nonn

Author

Clark Kimberling, Jul 20 2020

Status

approved