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A215478 Semiprimes with digits in descending order that differ exactly by 1. 1
4, 6, 9, 10, 21, 65, 87, 321, 543, 4321, 8765, 98765, 654321, 876543, 7654321 (list; graph; refs; listen; history; text; internal format)
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

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Last modified March 28 20:01 EDT 2020. Contains 333103 sequences. (Running on oeis4.)