A215477 Semiprimes with consecutive (ascending) digits.

4, 6, 9, 34, 123, 789, 901, 1234, 34567, 56789, 901234, 1234567, 7890123, 567890123, 12345678901, 345678901234567, 4567890123456789, 12345678901234567, 890123456789012345, 3456789012345678901, 456789012345678901234, 123456789012345678901234567, 1234567890123456789012345678901, 23456789012345678901234567890123

Offset: 1

Author

Jonathan Vos Post, Aug 12 2012

Keywords

• nonn
• base

Comments

This is to semiprimes A001358 as A006055 is to primes.

Examples

			a(9) = 34567 because it is semiprime 13 * 2659, and (3,4,5,6,7) are consecutive ascending digits.
		

Crossrefs

Cf. A001358, A118697, A059043, A006055, A161760.

Programs

Maple

R:= 4, 6, 9: V:= [$1..9]:
for d from 2 to 50 do
V:= map(n -> 10*n + ((n+1) mod 10), V);
W:= select(t -> numtheory:-bigomega(t)=2, V);
R:= R, op(W);
od:
R; # Robert Israel, Nov 15 2023

Extensions

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