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A257842
Semiprimes p*q such that R(p*q) = R(p)*R(q), where R = A004086 = reverse digits.
1
4, 6, 9, 22, 26, 33, 39, 46, 55, 62, 69, 77, 82, 86, 93, 121, 143, 169, 187, 202, 206, 226, 253, 262, 299, 303, 309, 339, 341, 393, 422, 446, 451, 466, 473, 482, 505, 583, 622, 626, 633, 662, 669, 671, 699, 707, 781, 802, 842, 862, 866, 886, 933, 939, 961
OFFSET
1,1
COMMENTS
A subsequence of A161600. Almost all terms with less than 4 digits are either multiples of 2 or 3 or of 11.
LINKS
MAPLE
N:= 1000: # to get all terms <= N
digrev:= proc(n) local L, i;
L:= convert(n, base, 10);
add(L[-i]*10^(i-1), i=1..nops(L))
end proc:
F:= proc(p, q) if digrev(p*q)=digrev(p)*digrev(q) then p*q else NULL fi end proc:
sort([seq(seq(F(Primes[i], q), q = select(`<=`, Primes[i..-1], N/Primes[i])), i=1..nops(Primes))]); # Robert Israel, May 14 2015
MATHEMATICA
f[n_]:=FactorInteger[n][[1, 1]]; g[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Select[Range@1000, PrimeOmega[#]==2&&g[f[#]*#/f[#]]==g[f[#]]*g[#/f[#]]&] (* Ivan N. Ianakiev, May 14 2015 *)
PROG
(PARI) is(n)=bigomega(n)==2&&!eval(concat(Vecrev(Str(n"-"vecmin(n=factor(n)[, 1])"*"vecmax(n)))))
CROSSREFS
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, May 11 2015
STATUS
approved