A074718 Least k such that floor(3^n/k) is prime.

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, 97, 38, 19, 55, 32, 23, 69, 110, 38, 114, 115, 31, 5, 15, 45, 29, 77, 7, 21, 63, 189, 37, 111, 226, 14, 42, 113, 44, 5, 15, 45, 135, 14, 38, 114, 137, 32, 37, 49, 147, 5, 15, 45, 79, 2

Offset

1,2

Comments

From Robert Israel, Jan 03 2017: (Start)

a(n+1) <= 3*a(n), with equality if and only if a(n+1) is divisible by 3.

For n > 1, a(n) <= floor(3^n/p) where p is the greatest prime <= 3^(n/2)-1.

a(n) = 2 if and only if n is in A028491. (End)

Links

Robert Israel, Table of n, a(n) for n = 1..2000

Maple

f:= proc(n) local t, k;
   t:= 3^n;
   for k from 2 to t/3 do if isprime(floor(t/k)) then return k fi od:
end proc:
f(1):= 1:
map(f, [$1..100]); # Robert Israel, Jan 03 2017

Prog

(PARI) a(n)=if(n<0, 0, k=1; while(isprime(floor(3^n/k))==0, k++); k)

Crossrefs

Cf. A028491.

Sequence in context: A248982 A333446 A289069 * A285457 A007812 A276225

Adjacent sequences: A074715 A074716 A074717 * A074719 A074720 A074721

Keyword

easy,nonn

Author

Benoit Cloitre, Sep 04 2002

Status

approved