A175520 - OEIS

%I #5 Mar 30 2012 18:52:39

%S 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 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 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 Number of distinct transpositions of digits (zeros and units) in n-th semiprime written in base 2.

%F a(n)=binomial ( (A000120(s)+A023416(s)), A000120(s) ), where s=semiprime(n).

%e a(1)=3 because (100,010,001) where semiprime(1)=3=100 (in base 2).

%Y Cf. A177687.

%K nonn,base

%O 1,1

%A _Juri-Stepan Gerasimov_, Jun 05 2010

%E a(56) corrected by _R. J. Mathar_, Jun 07 2010