A178043 n-th semiprime minus number of distinct transpositions of digits (zeros and units) in n-th semiprime written in base 2.

1, 3, 3, 4, 10, 14, 11, 12, 15, 16, 18, 19, 15, 18, 24, 31, 29, 36, 49, 42, 43, 56, 44, 34, 39, 42, 47, 50, 51, 66, 70, 72, 73, 88, 71, 104, 94, 97, 112, 100, 101, 116, 101, 77, 78, 71, 72, 87, 89, 90, 99, 102, 131, 105, 96, 99, 107, 108, 155, 129, 159, 138, 131, 132, 147

Offset: 1

Author

Juri-Stepan Gerasimov, May 17 2010

Keywords

• nonn
• base

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A001358, A122466.

Programs

Mathematica

#-Length[Permutations[IntegerDigits[#,2]]]&/@Select[Range[ 2,300], PrimeOmega[ #] == 2&] (* Harvey P. Dale, Jun 21 2014 *)

Formula

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

Extensions

More terms from R. J. Mathar, May 28 2010