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

1,1

Comments

This is to semiprimes A001358 as A006055 is to primes.

Links

Table of n, a(n) for n=1..24.

Example

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

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

Crossrefs

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

Sequence in context: A338378 A081614 A192220 * A292154 A291717 A303699

Adjacent sequences: A215474 A215475 A215476 * A215478 A215479 A215480

Keyword

nonn,base

Author

Jonathan Vos Post, Aug 12 2012

Extensions

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

Status

approved