A215478 - OEIS

%I #11 Jul 10 2014 10:17:11

%S 4,6,9,10,21,65,87,321,543,4321,8765,98765,654321,876543,7654321

%N Semiprimes with digits in descending order that differ exactly by 1.

%C This is to semiprimes A001358 as A052016 is to primes A000040.

%e 4321 is in the sequence because it is semiprime 4321 = 29 * 149, and (4,3,2,1) are descending consecutive digits.

%t Sort[Flatten[Table[Select[FromDigits/@Partition[Range[9,0,-1],n,1], PrimeOmega[#] == 2&],{n,10}]]] (* _Harvey P. Dale_, Jul 10 2014 *)

%Y Cf. A001358, A006055, A052016, A215477.

%K nonn,base,easy,full,fini

%O 1,1

%A _Jonathan Vos Post_, Aug 12 2012

%E Corrected and extended by _R. J. Mathar_, Aug 13 2012