A118697 Semiprimes with consecutive digits.

4, 6, 9, 10, 21, 34, 65, 87, 123, 321, 543, 789, 901, 1234, 4321, 8765, 34567, 56789, 98765, 654321, 876543, 901234, 1234567, 7654321, 7890123, 10987654, 21098765, 567890123, 12345678901, 54321098765, 76543210987, 87654321098

Offset

1,1

Comments

Digits can be in ascending or descending order. After 9 comes 0.

The number of semiprimes with n consecutive digits: 3,5,5,3,3,3,3,2,1,0,4,0,2,0,2,1,1,1,1,0,1,2,0,1,0,0,2,1,0,0,1,1,1,0,0,0,0,2,1,0,1,0,0,0,2,1,0,0,0,0,..., . - Robert G. Wilson v, Jun 16 2006

Links

Table of n, a(n) for n=1..32.

Mathematica

spQ[n_] := Plus @@ Last /@ FactorInteger@n == 2; f[n_] := Block[{u = Range@n, d = Reverse@ Range@n, t = Table[1, {n}]}, Select[ Drop[ Union@ Flatten@ Table[ FromDigits /@ Mod[{u, d} + {i*t, i*t}, 10], {i, 10}], 2], spQ@# &]]; Array[f, 13] // Flatten (* Robert G. Wilson v, Jun 16 2006 *)

Crossrefs

Cf. A001358.

Sequence in context: A108635 A071964 A135257 * A118695 A235108 A215478

Adjacent sequences: A118694 A118695 A118696 * A118698 A118699 A118700

Keyword

base,nonn

Author

Luc Stevens (lms022(AT)yahoo.com), May 20 2006

Extensions

More terms from Robert G. Wilson v, Jun 16 2006

Status

approved