A174173 Semiprimes with consecutive digits starting with 1.

123, 1234, 1234567, 12345678901, 12345678901234567, 123456789012345678901234567, 1234567890123456789012345678901

Offset

1,1

Links

Table of n, a(n) for n=1..7.

Example

123=3*41, 1234=2*617, 1234567=127*9721, 12345678901=857*14405693,..

Mathematica

f1[n_]:=Last@RealDigits[n, 10][[1]]; f2[n_]:=Last/@FactorInteger[n]=={1, 1}; lst={}; lst2={}; Do[AppendTo[lst, f1[n]]; a=FromDigits[lst]; If[f2[a], (*Print[a]; *)AppendTo[lst2, a]], {n, 43}]; lst2
Select[Table[FromDigits[PadRight[{}, n, Join[Range[9], {0}]]], {n, 40}], PrimeOmega[ #]==2&] (* Harvey P. Dale, Jun 07 2013 *)

Crossrefs

Sequence in context: A135475 A233119 A167231 * A116935 A265983 A103504

Adjacent sequences: A174170 A174171 A174172 * A174174 A174175 A174176

Keyword

nonn,base

Author

Vladimir Joseph Stephan Orlovsky, Mar 11 2010

Extensions

Next terms have 124, 157, 247, ... decimal digits.

Status

approved