A215478 Semiprimes with digits in descending order that differ exactly by 1.

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

Offset: 1

Author

Jonathan Vos Post, Aug 12 2012

Keywords

• nonn
• base
• easy
• full
• fini

Comments

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

Examples

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

Crossrefs

Cf. A001358, A006055, A052016, A215477.

Programs

Mathematica

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

Extensions

Corrected and extended by R. J. Mathar, Aug 13 2012