OFFSET
1,2
COMMENTS
Also, with offset 0, product of first n odd primes. - N. J. A. Sloane, Feb 26 2017
Identical to A002110(n)/2, n>=1.
a(n+1) is the least odd number with exactly n distinct prime divisors. - Labos Elemer, Mar 24 2003
Also, odd numbers n for which sigma(n)*phi(n)/n^2 reaches a new record low, monotonically decreasing to the lower bound 8/Pi^2. - M. F. Hasler, Jul 08 2025
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..200
FORMULA
a(n) = A002110(n)/2.
From Antti Karttunen, Feb 06 2024: (Start)
(End)
a(n) = A000040(n)*a(n-1) for n > 1, a(1) = 1. - M. F. Hasler, Jul 09 2025
MAPLE
a:=n->mul(ithprime(j), j=2..n):seq(a(n), n=1..17); # Zerinvary Lajos, Aug 24 2008
MATHEMATICA
Rest[ FoldList[ Times, 1, Prime[ Range[ 18]] ]]/2 (* Robert G. Wilson v, Feb 17 2004 *)
FoldList[Times, 1, Prime[Range[2, 18]]] (* Zak Seidov, Jan 26 2009 *)
PROG
(PARI) a(n) = prod(k=2, n, prime(k)) \\ Michel Marcus, Mar 25 2017, simplified by M. F. Hasler, Jul 09 2025
(Python)
from sympy import primorial
def A070826(n): return primorial(n)>>1 # Chai Wah Wu, Jul 21 2022
CROSSREFS
Range of A196529.
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, May 10 2002
EXTENSIONS
Formula corrected by Gary Detlefs, Dec 07 2011
STATUS
approved
