OFFSET
1,2
COMMENTS
Or, lower central divisor of n-th primorial.
Subsequence of A005117 (squarefree numbers). - Michel Marcus, Feb 22 2016
LINKS
Max Alekseyev, Table of n, a(n) for n = 1..70
FORMULA
EXAMPLE
n = 8: q(8) = 2*3*5*7*11*13*17*19 = 9699690. Its 128th and 129th divisors are {3094, 3135}: a(8) = 3094 and 3094 < A000196(9699690) = 3114 < 3135. [Corrected by Colin Barker, Oct 22 2010]
2*3*5*7 = 210 = 14*15 with difference of 1, so a(4) = 14.
MAPLE
F:= proc(n) local P, N, M;
P:= {seq(ithprime(i), i=1..n)};
N:= floor(sqrt(convert(P, `*`)));
M:= map(convert, combinat:-powerset(P), `*`);
max(select(`<=`, M, N))
end proc:
map(F, [$1..20]); # Robert Israel, Feb 22 2016
MATHEMATICA
a[n_] := (m = Times @@ Prime[Range[n]] ; dd = Divisors[m]; dd[[Length[dd]/2 // Floor]]); Table[Print[an = a[n]]; an, {n, 1, 25}] (* Jean-François Alcover, Oct 15 2016 *)
PROG
(PARI) a(n) = my(m=prod(i=1, n, prime(i))); divisors(m)[numdiv(m)\2]; \\ Michel Marcus, Feb 22 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Apr 27 2001
EXTENSIONS
More terms from Ed Pegg Jr, May 28 2001
a(16)-a(23) computed by Jud McCranie, Apr 15 2000
a(24) and a(25) from Robert Israel, Feb 22 2016
a(25) corrected by Jean-François Alcover, Oct 15 2016
a(26)-a(33) in b-file from Amiram Eldar, Apr 09 2020
Up to a(38) using b-file of A060796, by M. F. Hasler, Mar 21 2022
a(39)-a(70) in b-file from Max Alekseyev, Apr 20 2022
STATUS
approved