A257842 Semiprimes p*q such that R(p*q) = R(p)*R(q), where R = A004086 = reverse digits.

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

Giovanni Resta, Table of n, a(n) for n = 1..10000

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

Cf. A161600, A071786, A004086.

Sequence in context: A032817 A357741 A115681 * A239307 A137253 A035135

Adjacent sequences: A257839 A257840 A257841 * A257843 A257844 A257845

Keyword

nonn,base

Author

M. F. Hasler, May 11 2015

Status

approved