This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A215478 #11 Jul 10 2014 10:17:11
%S A215478 4,6,9,10,21,65,87,321,543,4321,8765,98765,654321,876543,7654321
%N A215478 Semiprimes with digits in descending order that differ exactly by 1.
%C A215478 This is to semiprimes A001358 as A052016 is to primes A000040.
%e A215478 4321 is in the sequence because it is semiprime 4321 = 29 * 149, and (4,3,2,1) are descending consecutive digits.
%t A215478 Sort[Flatten[Table[Select[FromDigits/@Partition[Range[9,0,-1],n,1], PrimeOmega[#] == 2&],{n,10}]]] (* _Harvey P. Dale_, Jul 10 2014 *)
%Y A215478 Cf. A001358, A006055, A052016, A215477.
%K A215478 nonn,base,easy,full,fini
%O A215478 1,1
%A A215478 _Jonathan Vos Post_, Aug 12 2012
%E A215478 Corrected and extended by _R. J. Mathar_, Aug 13 2012