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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
OFFSET
1,1
LINKS
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
KEYWORD
nonn,base
AUTHOR
Giovanni Resta, Jan 31 2006
STATUS
approved