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

1,1

Comments

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

Links

Table of n, a(n) for n=1..15.

Example

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

Mathematica

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

Crossrefs

Cf. A001358, A006055, A052016, A215477.

Sequence in context: A118697 A118695 A235108 * A118693 A084668 A085842

Adjacent sequences: A215475 A215476 A215477 * A215479 A215480 A215481

Keyword

nonn,base,easy,full,fini

Author

Jonathan Vos Post, Aug 12 2012

Extensions

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

Status

approved