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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A115688 Semiprimes (A001358) whose digit reversal is a powerful(1) number (A001694). 2
4, 9, 10, 46, 94, 121, 169, 215, 526, 869, 961, 982, 1042, 1273, 1405, 1843, 2918, 3194, 4069, 4633, 5213, 5221, 5758, 6313, 6511, 6937, 8045, 8402, 8651, 8882, 9235, 9481, 9586, 9886, 10201, 10609, 12538, 12769, 14023, 16171, 16327, 16582 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Robert Israel, Table of n, a(n) for n = 1..908

EXAMPLE

869=11*79 is semiprime and 968=2^3*11^2 is powerful.

MAPLE

N:= 99999:

S:= {1}:

p:= 1:

do

  p:= nextprime(p);

  if p^2 > N then break fi;

  S:= S union map(t -> seq(t*p^j, j=2..floor(log[p](N/t))), S);

od:

digrev:= proc(x) local L;

  L:= convert(x, base, 10);

  add(L[-i]*10^(i-1), i=1..nops(L))

end proc:

sort(convert({10} union select(t -> numtheory:-bigomega(t)=2, map(digrev, select(t -> t mod 10 <> 0, S))), list)); # Robert Israel, Dec 03 2019

CROSSREFS

Cf. A001358, A001694, A115687.

Sequence in context: A178360 A197125 A197129 * A115710 A082749 A220427

Adjacent sequences:  A115685 A115686 A115687 * A115689 A115690 A115691

KEYWORD

nonn,base

AUTHOR

Giovanni Resta, Jan 31 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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 24 22:57 EST 2020. Contains 332216 sequences. (Running on oeis4.)