OFFSET
1,1
COMMENTS
All positive multiples of 4 are in this sequence other than 4. Similarly, all numbers which are 0 or 8 mod 9 are in this sequence and all multiples of 25 are in this sequence. - Charles R Greathouse IV, Sep 20 2012
LINKS
Patrick De Geest, Normal Smarandache Concatenated Numbers, Prime factors from 1 up to n
M. Fleuren, Factors and primes of Smarandache sequences.
M. Fleuren, Smarandache Factors and Reverse factors
Carlos Rivera, Puzzle 8. Primes by Listing, The Prime Puzzles & Problems Connection.
FORMULA
n < a(n) < 2.4n for n > 10. The upper constant can be lowered to 25/11 with more work. - Charles R Greathouse IV, Sep 20 2012
EXAMPLE
6 is a term because 123456 = 2*2*2*2*2*2*3*643 = 2^6*3*643, not squarefree.
MATHEMATICA
t={}; m=1; Do[m=FromDigits[Flatten[IntegerDigits[{m, n}]]]; If[!SquareFreeQ[m], AppendTo[t, n]], {n, 2, 32}]; t (* Jayanta Basu, May 30 2013 *)
Module[{nn=120, cct}, cct=Table[If[SquareFreeQ[FromDigits[Flatten[IntegerDigits/@Range[ n]]]], 0, 1], {n, nn}]; Position[cct, _?(#==1&)]]//Flatten (* Harvey P. Dale, Sep 20 2023 *)
PROG
(Magma) a:=[]; c:=1; for n in [2..55] do c:=c*10^#Intseq(n)+n; if not IsSquarefree(c) then Append(~a, n); end if; end for; a; // Marius A. Burtea, Oct 15 2019
(PARI) conc(n) = my(s=""); for(k=1, n, s=Str(s, k)); eval(s); \\ A007908
isok(n) = ! issquarefree(conc(n)); \\ Michel Marcus, Oct 16 2019
CROSSREFS
KEYWORD
nonn,base,hard,less
AUTHOR
Patrick De Geest, Aug 15 1999
EXTENSIONS
Corrected by Charles R Greathouse IV, Sep 20 2012
More terms from Sean A. Irvine, Aug 16 2021
STATUS
approved