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 A175520 #5 Mar 30 2012 18:52:39 %S A175520 3,3,6,6,4,1,10,10,10,10,15,15,20,20,15,15,20,15,6,15,15,6,21,35,35, %T A175520 35,35,35,35,21,21,21,21,7,35,7,21,21,7,21,21,7,28,56,56,70,70,56,56, %U A175520 56,56,56,28,56,70,70,70,70,28,56,28,56,70,70,56,56,56,70,56,56,28,56,56,28 %N A175520 Number of distinct transpositions of digits (zeros and units) in n-th semiprime written in base 2. %F A175520 a(n)=binomial ( (A000120(s)+A023416(s)), A000120(s) ), where s=semiprime(n). %e A175520 a(1)=3 because (100,010,001) where semiprime(1)=3=100 (in base 2). %Y A175520 Cf. A177687. %K A175520 nonn,base %O A175520 1,1 %A A175520 _Juri-Stepan Gerasimov_, Jun 05 2010 %E A175520 a(56) corrected by _R. J. Mathar_, Jun 07 2010