OFFSET
1,1
COMMENTS
For large n, the ratio prime(n)/a(n) tends to e (very slowly). This was conjectured by Anton Vrba in 2010 (see Rivera, 2010) and proved by Sandor and Verroken (2011). Tighter bounds and asymptotics for a(n) are proved in the note "On the geometric mean of the first n primes" (2016) (see links). Better formulas prime(n)/a(n) ~ exp(1 + several terms A233824(k) / log^k(prime(n))) exist for larger n; see examples in the formula section. - Alexei Kourbatov, Feb 27 2016.
LINKS
Harry J. Smith, Table of n, a(n) for n=1..1000
Christian Axler, On the arithmetic and geometric means of the prime numbers, arXiv:1609.07934 [math.NT], 2016.
A. Kourbatov, On the geometric mean of the first n primes, arXiv:1603.00855 [math.NT], 2016.
C. Rivera, ed. Conjecture 67. Primes and e, 2010.
J. Sandor and A. Verroken, On a limit involving the product of prime numbers, Notes Number Theory Discrete Math. 17 (2011), No. 2, 1-3.
FORMULA
From Alexei Kourbatov, Feb 22 2016: (Start)
a(n) ~ prime(n)/exp(1 + 1/log(prime(n)) + O(1/log^2(prime(n)))).
a(n) ~ prime(n)/e (this approximation is poor).
a(n) ~ prime(n)/exp(1 + 1/log(prime(n))).
a(n) ~ prime(n)/exp(1 + 1/log(prime(n)) + 3/log^2(prime(n))).
a(n) ~ prime(n)/exp(1 + 1/log(prime(n)) + 3/log^2(prime(n)) + 13/log^3(prime(n))).
a(n) < (1/2)*prime(n) for n>3.
(End)
a(n) = floor(A002110(n)^(1/n)). - Michel Marcus, Feb 22 2016
EXAMPLE
a(5) = floor( (2*3*5*7*11)^(1/5) ) = 4.
MAPLE
P:= 1:
A[0]:= 1:
for n from 1 to 100 do
P:= ithprime(n)*P;
for k from A[n-1] while (k+1)^n <= P do od:
A[n]:= k;
od:
seq(A[i], i=1..100); # Robert Israel, Feb 22 2016
MATHEMATICA
With[{pl=Prime[Range[80]]}, Table[IntegerPart[GeometricMean[Take[pl, n]]], {n, 80}]] (* Harvey P. Dale, Mar 31 2012 *)
PROG
(PARI) { default(realprecision, 100); p=1; for (n=1, 1000, p*=prime(n); write("b062049.txt", n, " ", p^(1/n)\1) ) } \\ Harry J. Smith, Jul 30 2009
CROSSREFS
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Jun 06 2001
EXTENSIONS
More terms from Larry Reeves (larryr(AT)acm.org) and Matthew Conroy, Jun 11 2001
STATUS
approved