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

%I A257842 #12 May 24 2015 04:13:28
%S A257842 4,6,9,22,26,33,39,46,55,62,69,77,82,86,93,121,143,169,187,202,206,
%T A257842 226,253,262,299,303,309,339,341,393,422,446,451,466,473,482,505,583,
%U A257842 622,626,633,662,669,671,699,707,781,802,842,862,866,886,933,939,961
%N A257842 Semiprimes p*q such that R(p*q) = R(p)*R(q), where R = A004086 = reverse digits.
%C A257842 A subsequence of A161600. Almost all terms with less than 4 digits are either multiples of 2 or 3 or of 11.
%H A257842 Giovanni Resta, <a href="/A257842/b257842.txt">Table of n, a(n) for n = 1..10000</a>
%p A257842 N:= 1000: # to get all terms <= N
%p A257842 digrev:= proc(n) local L,i;
%p A257842 L:= convert(n,base,10);
%p A257842 add(L[-i]*10^(i-1),i=1..nops(L))
%p A257842 end proc:
%p A257842 F:= proc(p,q) if digrev(p*q)=digrev(p)*digrev(q) then p*q else NULL fi end proc:
%p A257842 sort([seq(seq(F(Primes[i],q), q = select(`<=`,Primes[i..-1],N/Primes[i])), i=1..nops(Primes))]); # _Robert Israel_, May 14 2015
%t A257842 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 *)
%o A257842 (PARI) is(n)=bigomega(n)==2&&!eval(concat(Vecrev(Str(n"-"vecmin(n=factor(n)[,1])"*"vecmax(n)))))
%Y A257842 Cf. A161600, A071786, A004086.
%K A257842 nonn,base
%O A257842 1,1
%A A257842 _M. F. Hasler_, May 11 2015