A215477 -id:A215477 - cp's OEIS frontend

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

Jonathan Vos Post, Aug 12 2012

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

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

Cf. A001358, A006055, A052016, A215477.

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

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

A245044 Semiprimes with digits in descending order that differ exactly by 2.

Original entry on oeis.org

4, 6, 9, 86, 753, 7531, 9753, 97531

Offset: 1

Vincenzo Librandi, Jul 10 2014

			7531 is in the sequence because it is semiprime 7531 = 17*443 and 7-5 = 5-3 = 3-1 = 2.
97531 is in the sequence because it is semiprime 97531 = 7*13933 and 9-7 = 7-5 = 5-3 = 3-1 = 2.

Cf. A215477, A215478.

Sort[Flatten[Table[Select[FromDigits/@Partition[Range[9, 0, -2], n, 1], PrimeOmega[#]==2 &], {n, 1000}]]]

for(d=1,5,for(s=1,11-2*d,t=sum(i=1,d,(s-2+2*i)*10^i)/10; if(bigomega(t)==2, print1(t", ")))) \\ Charles R Greathouse IV, Jul 16 2014

a(1), a(2), a(4) from Charles R Greathouse IV, Jul 16 2014

cp's OEIS Frontend

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

Views

Author

Keywords

Comments

Examples

Crossrefs

Programs

Mathematica

Extensions