A256421 Odd numbers and twice primes, sorted.
1, 3, 4, 5, 6, 7, 9, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 25, 26, 27, 29, 31, 33, 34, 35, 37, 38, 39, 41, 43, 45, 46, 47, 49, 51, 53, 55, 57, 58, 59, 61, 62, 63, 65, 67, 69, 71, 73, 74, 75, 77, 79, 81, 82, 83, 85, 86, 87, 89, 91, 93, 94
Offset: 1
Keywords
Links
- Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
Crossrefs
Cf. A256420.
Programs
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Mathematica
Select[Range@94,PrimeQ[#/2]||OddQ[#]&] (* Ivan N. Ianakiev, Apr 08 2015 *)
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PARI
lista(nn) = nb = primepi(nn)\2; vecsort(setunion(vector(prime(nb), n, 2*n-1), 2*primes(nb))); \\ Michel Marcus, Apr 07 2015
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PARI
list(lim)=setunion(vector((lim+1)\2,i,2*i-1), 2*primes([2,lim])) \\ Charles R Greathouse IV, Apr 07 2015
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Python
from sympy import primepi def A256421(n): def f(x): return int(n+(m:=x>>1)-primepi(m)) m, k = n, f(n) while m != k: m, k = k, f(k) return m # Chai Wah Wu, Oct 15 2024
Formula
a(n) ~ 2n. - Charles R Greathouse IV, Apr 07 2015