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A097393 Emirpimes: numbers n such that n and its reversal are distinct semiprimes. 30
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 (list; graph; refs; listen; history; text; internal format)
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

Select[Range[500], PrimeOmega[#] == 2 && PrimeOmega[FromDigits[Reverse[IntegerDigits[#]]]] == 2 &] (* Alonso del Arte, Oct 08 2012 *)

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

Cf. A001358, A097394.

Sequence in context: A189045 A032609 A050699 * A050700 A263108 A274182

Adjacent sequences: A097390 A097391 A097392 * A097394 A097395 A097396

KEYWORD

nonn,base

AUTHOR

Jonathan Vos Post

STATUS

approved

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Last modified December 6 01:03 EST 2022. Contains 358594 sequences. (Running on oeis4.)