|
%I #8 Mar 30 2016 12:17:03
%S 5,161,164865,21609185281,1450167876173430785,
%T 199309555259989211062227435521,
%U 224402609700066672987369178785813291654447105
%N Decimal value of binary number whose n-th 1 is followed by n^2 0's.
%C After a(1) = 5, when is a(n) prime?
%F a(n) = SUM[i=1,n] 2^(n+1).
%e a(1) = 5 because 101 (base 2) = 5 (base 10).
%e a(2) = 161 because 10100001 (base 2) = 161 (base 10).
%e a(3) = 164865 because 101000010000000001 (base 2) = 164865 (base 10).
%t Table[FromDigits[Flatten[Join[{1},Table[Join[PadRight[{},n^2,0],{1}],{n,i}]]],2],{i,8}] (* _Harvey P. Dale_, Mar 30 2016 *)
%K base,easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Apr 16 2006
| 