|
%I #12 Jan 04 2017 02:45:32
%S 1,3,2,6,14,23,2,6,11,14,14,32,2,6,5,15,14,42,5,7,21,63,25,61,19,53,
%T 97,38,19,55,32,23,69,110,38,114,115,31,5,15,45,29,77,7,21,63,189,37,
%U 111,226,14,42,113,44,5,15,45,135,14,38,114,137,32,37,49,147,5,15,45,79,2
%N Least k such that floor(3^n/k) is prime.
%C From _Robert Israel_, Jan 03 2017: (Start)
%C a(n+1) <= 3*a(n), with equality if and only if a(n+1) is divisible by 3.
%C For n > 1, a(n) <= floor(3^n/p) where p is the greatest prime <= 3^(n/2)-1.
%C a(n) = 2 if and only if n is in A028491. (End)
%H Robert Israel, <a href="/A074718/b074718.txt">Table of n, a(n) for n = 1..2000</a>
%p f:= proc(n) local t,k;
%p t:= 3^n;
%p for k from 2 to t/3 do if isprime(floor(t/k)) then return k fi od:
%p end proc:
%p f(1):= 1:
%p map(f, [$1..100]); # _Robert Israel_, Jan 03 2017
%o (PARI) a(n)=if(n<0,0,k=1; while(isprime(floor(3^n/k))==0,k++); k)
%Y Cf. A028491.
%K easy,nonn
%O 1,2
%A _Benoit Cloitre_, Sep 04 2002
|
