OFFSET
1,2
COMMENTS
Except for 1, this sequence is a subsequence of A280928. More specifically, members of A280928 are also members of this sequence if and only if they are semiprime. - Ely Golden, Jan 11 2017
This sequence has no equivalent in odd bases. This is because any equivalent of A280928 in an odd base must have all terms having at least 3 prime factors. - Ely Golden, Jan 11 2017
All entries other than 1 are congruent to 4 mod 9, because p*q == p + q mod 9 (with p and q not both divisible by 3) implies p*q == 4 mod 9. - Robert Israel, May 05 2014
LINKS
Robert Israel, Table of n, a(n) for n = 1..145
EXAMPLE
1255 = 5*251, 12955 = 5*2591, 17482 = 2*8741, 100255 = 5*20051, 146137=317*461, etc.
MAPLE
filter:= proc(n) local F, p, q, Ln, Lpq;
F:= ifactors(n)[2];
if nops(F) > 2 or convert(F, `+`)[2]<>2 then return false fi;
p:= F[1][1];
if nops(F) = 2 then q:= F[2][1] else q:= F[1][1] fi;
Ln:= sort(convert(n, base, 10));
Lpq:= sort([op(convert(p, base, 10)), op(convert(q, base, 10))]);
evalb(Ln = Lpq);
end proc:
filter(1):= true:
A080718:= select(filter, [1, seq(4+9*i, i=1..10^6)]); # Robert Israel, May 04 2014
MATHEMATICA
ptpQ[n_]:=Module[{sidn=Sort[IntegerDigits[n]], fi=Transpose[ FactorInteger[ n]]}, fi[[2]]=={1, 1}&&Sort[Flatten[ IntegerDigits/@ fi[[1]]]]==sidn]; Join[{1}, Select[Range[4, 550000, 9], ptpQ]] (* Harvey P. Dale, Jun 22 2014 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Jeff Heleen, Mar 06 2003
EXTENSIONS
Edited by N. J. A. Sloane, Jan 03 2009
Incorrect entry 163797 removed by Robert Israel, May 04 2014
STATUS
approved