A074720 Least k such that floor(3^n/2^k) is prime.

2, 1, 4, 5, 6, 1, 11, 6, 7, 4, 5, 1, 9, 6, 8, 21, 8, 4, 25, 12, 20, 13, 30, 17, 6, 13, 10, 13, 19, 5, 12, 34, 33, 37, 16, 39, 35, 13, 38, 30, 28, 20, 53, 16, 60, 24, 40, 43, 34, 19, 23, 32, 63, 59, 19, 22, 27, 56, 86, 14, 29, 5, 53, 13, 15, 63, 19, 7, 88, 1, 87, 46, 22, 51, 25, 30

Offset: 2

Author

Benoit Cloitre, Sep 04 2002

Keywords

• easy
• nonn

Comments

From Robert Israel, Jan 04 2017: (Start)

a(n) <= A056576(n) - 1.

a(n) = 1 for n in A028491. (End)

Links

• Robert Israel, Table of n, a(n) for n = 2..2000

Programs

Maple

f:= proc(n) local t, k;
   t:= 3^n;
   for k from 1 do t:= t/2; if isprime(floor(t)) then return k fi od:
end proc:
map(f, [$2..100]); # Robert Israel, Jan 04 2017

Mathematica

lk[n_]:=Module[{k=1,n3=3^n},While[!PrimeQ[Floor[n3/2^k]],k++];k]; Array[lk,80,2] (* Harvey P. Dale, Feb 24 2013 *)

PARI

a(n)=if(n<0,0,k=1; while(isprime(floor(3^n/2^k)) == 0,k++); k)