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A115688

Semiprimes (A001358) whose digit reversal is a powerful(1) number (A001694).

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

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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:

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

URL: https://oeis.org/A115688

⇱ A115688 - OEIS