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

%I A074720 #12 Jan 04 2017 19:40:43
%S A074720 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,
%T A074720 19,5,12,34,33,37,16,39,35,13,38,30,28,20,53,16,60,24,40,43,34,19,23,
%U A074720 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
%N A074720 Least k such that floor(3^n/2^k) is prime.
%C A074720 From _Robert Israel_, Jan 04 2017: (Start)
%C A074720 a(n) <= A056576(n) - 1.
%C A074720 a(n) = 1 for n in A028491. (End)
%H A074720 Robert Israel, <a href="/A074720/b074720.txt">Table of n, a(n) for n = 2..2000</a>
%p A074720 f:= proc(n) local t, k;
%p A074720    t:= 3^n;
%p A074720    for k from 1 do t:= t/2; if isprime(floor(t)) then return k fi od:
%p A074720 end proc:
%p A074720 map(f, [$2..100]); # _Robert Israel_, Jan 04 2017
%t A074720 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 *)
%o A074720 (PARI) a(n)=if(n<0,0,k=1; while(isprime(floor(3^n/2^k)) == 0,k++); k)
%K A074720 easy,nonn
%O A074720 2,1
%A A074720 _Benoit Cloitre_, Sep 04 2002