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A097393
Emirpimes: numbers n such that n and its reversal are distinct semiprimes.
32
15, 26, 39, 49, 51, 58, 62, 85, 93, 94, 115, 122, 123, 129, 143, 155, 158, 159, 169, 177, 178, 183, 185, 187, 203, 205, 221, 226, 265, 289, 302, 314, 319, 321, 326, 327, 329, 335, 339, 341, 355, 381, 394, 398, 413, 415, 437, 493, 497, 502, 511, 514, 533
OFFSET
1,1
COMMENTS
Computed by Eric W. Weisstein, Aug 13 2004.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Emirpimes
EXAMPLE
26 is a semiprime, as it is 2 * 13, and so is 62 = 2 * 31. 26 and 62 are therefore both in the sequence.
MAPLE
isA097393 := proc(n)
local R ;
R := digrev(n) ;
if R <> n then
if numtheory[bigomega](R) = 2 and numtheory[bigomega](n) = 2 then
return true;
else
false;
end if;
else
false;
end if;
end proc:
for n from 1 to 500 do
if isA097393(n) then
printf("%d, ", n) ;
end if;
end do: # R. J. Mathar, Apr 05 2012
MATHEMATICA
Cases[{#, IntegerReverse@#} & /@ DeleteCases[Range@5000, _?PalindromeQ], {_?(PrimeOmega@# == 2 &) ..}][[All, 1]] (* Hans Rudolf Widmer, Jan 07 2024 *)
PROG
(PARI) rev(n)=subst(Polrev(digits(n)), 'x, 10)
issemi(n)=bigomega(n)==2
list(lim)=my(v=List(), r); forprime(p=2, lim\2, forprime(q=2, min(lim\p, p), r=rev(p*q); if(issemi(r)&&r!=p*q, listput(v, p*q)))); Set(v) \\ Charles R Greathouse IV, Jan 27 2015
(Python)
from sympy import factorint
from itertools import islice
def sp(n): f = factorint(n); return sum(f[p] for p in f) == 2
def ok(n): r = int(str(n)[::-1]); return r != n and sp(n) and sp(r)
print([k for k in range(534) if ok(k)]) # Michael S. Branicky, Jul 03 2022
CROSSREFS
Equals A085751 \ A046328.
Sequence in context: A366961 A032609 A050699 * A050700 A373731 A263108
KEYWORD
nonn,base
STATUS
approved