The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A118697 Semiprimes with consecutive digits. 2
 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 (list; graph; refs; listen; history; text; internal format)
 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 6 14:08 EDT 2020. Contains 333276 sequences. (Running on oeis4.)