This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A102029 #19 Jul 11 2015 10:20:48
%S A102029 4,6,14,15,55,95,247,447,511,1535,2047,7167,12287,32255,49151,98303,
%T A102029 196607,393215,983039,1572863,3145727,6291455,8388607,33423359,
%U A102029 50331647,117440511,201326591,528482303,805306367,1879048191,3221225471
%N A102029 Smallest semiprime with Hamming weight n (i.e., smallest semiprime with exactly n ones when written in binary), or -1 if no such number exists.
%C A102029 Semiprime analog of A061712. Extended by _Stefan Steinerberger_. Includes the subset Mersenne semiprimes A092561.
%H A102029 Donovan Johnson, <a href="/A102029/b102029.txt">Table of n, a(n) for n = 1..250</a>
%e A102029 a(1) = 4 because the first semiprime A001358(1) is 4 (base 10) which is written 100 in binary, the latter representation having exactly 1 one.
%e A102029 a(2) = 6 since A001358(2) = 6 = 110 (base 2) has exactly 2 ones.
%e A102029 a(4) = 15 since A001358(6) = 15 = 1111 (base 2) has exactly 4 ones and, as it also has no zeros, is the smallest of the Mersenne semiprimes.
%t A102029 Join[{4},Table[SelectFirst[Sort[FromDigits[#,2]&/@Permutations[ Join[ PadRight[{}, n,1],{0}]]],PrimeOmega[#]==2&],{n,2,40}]] (* _Harvey P. Dale_, Feb 06 2015 *)
%Y A102029 Cf. A000043, A000120, A000337, A000668, A001358, A007088, A061712, A085724, A089226, A089998, A089999, A091991, A092558, A092559, A092561, A092562, A081093, A102782, A110472, A110699, A110700.
%K A102029 easy,base,nonn
%O A102029 1,1
%A A102029 _Jonathan Vos Post_, Jun 23 2007