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A174173 Semiprimes with consecutive digits starting with 1. 0
123, 1234, 1234567, 12345678901, 12345678901234567, 123456789012345678901234567, 1234567890123456789012345678901 (list; graph; refs; listen; history; text; internal format)
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: A077379 A135475 A233119 * 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

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Last modified September 22 01:18 EDT 2021. Contains 347605 sequences. (Running on oeis4.)