A261871 - OEIS

%I #35 Dec 15 2017 14:28:44

%S 3,7,9,15,21,27,31,33,35,39,45,49,51,57,63,69,75,77,81,87,91,93,99,

%T 105,111,117,119,123,127,129,133,135,141,147,153,155,159,161,165,171,

%U 175,177,183,189,195,201,203,207,213,217,219,225,231,237,243,245,249,255,259,261,267,273,279,285,287,291,297,301

%N Numbers of the form (2*j-1)*(2^k-1); j>=1, k>=2.

%C Odd numbers complementary to A185208.

%C Lim_{n->inf.} a(n)/n > 6/(1 + Sum_{j>=1} (2/(2^(2j+1)-1))) ~ 4.375745.

%H Charles R Greathouse IV, <a href="/A261871/b261871.txt">Table of n, a(n) for n = 1..10000</a>

%F 2n < a(n) < 5n. For n > 51, 4.3n < a(n) < 4.5n. - _Charles R Greathouse IV_, Sep 05 2015

%t lmt = 310; Take[ Union@ Flatten@ Table[ (2j - 1)(2^k - 1), {j, lmt/4}, {k, 2, 1 + Log2[ lmt/(2j)] }], 68] (* _Michael De Vlieger_, Sep 04 2015 *) (* and modified by _Robert G. Wilson v_, Sep 05 2015 *)

%o (PARI) list(lim)=my(v=List(),t); for(k=2,logint(lim\1+1,2), t=2^k-1; forstep(j=1,lim\t,2, listput(v,t*j))); Set(v) \\ _Charles R Greathouse IV_, Sep 05 2015

%Y Cf. A185208.

%Y Note that A191131, A261524, A261871, and A282572 are very similar and easily confused with each other.

%K nonn,easy

%O 1,1

%A _Bob Selcoe_, Sep 04 2015