cp's OEIS Frontend

A210625 Least semiprime dividing digit reversal of n, or 0 if no such factor.

%I A210625 #32 Jul 10 2015 20:13:05
%S A210625 0,0,0,4,0,6,0,4,9,0,0,21,0,0,51,0,0,9,91,0,4,22,4,6,4,62,4,82,4,0,0,
%T A210625 0,33,0,0,9,0,0,93,4,14,4,34,4,6,4,74,4,94,0,15,25,35,9,55,65,15,85,
%U A210625 95,6,4,26,4,46,4,6,4,86,4,0,0,9,0,0,57,0,77,87
%N A210625 Least semiprime dividing digit reversal of n, or 0 if no such factor.
%C A210625 Roughly the analog of A209190 (least prime factor of reversal of digits), but with semiprimes (A001358) instead of primes (A000040).
%H A210625 Alois P. Heinz, <a href="/A210625/b210625.txt">Table of n, a(n) for n = 1..10000</a>
%F A210625 a(n) = A210615(R(n)) = A210615(A004086(n)).
%F A210625 a(p) = 0 iff p in (A004087 union A011557). - _Alois P. Heinz_, Mar 28 2012
%e A210625 a(12) = min {k such that k|R(12) and k = p*q for primes p and q (not necessarily distinct)} = min {k, k|21 and k semiprime} = 21 = 3*7.
%e A210625 a(42) = min {k, k|24 and k semiprime} = min {4,6} = 4 = 2*2.
%p A210625 r:= proc(n) option remember; local q;
%p A210625       `if`(n<10, n, irem(n, 10, 'q') *10^(length(n)-1)+r(q))
%p A210625     end:
%p A210625 a:= proc(n) local m, k;
%p A210625       m:= r(n);
%p A210625       for k from 4 to m do
%p A210625          if irem(m, k)=0 and not isprime(k) and
%p A210625             add(i[2], i=ifactors(k)[2])=2 then return k fi
%p A210625       od; 0
%p A210625     end:
%p A210625 seq(a(n), n=1..100);  # _Alois P. Heinz_, Mar 26 2012
%t A210625 spd[n_]:=Module[{sps=Select[Divisors[FromDigits[Reverse[ IntegerDigits[n]]]], PrimeOmega[#] == 2&,1]},If[sps=={},0,First[sps]]]; Array[spd,80] (* _Harvey P. Dale_, Aug 12 2012 *)
%Y A210625 Cf. A001358, A004086, A004087, A011557, A209190, A210615.
%K A210625 nonn,base,look,easy
%O A210625 1,4
%A A210625 _Jonathan Vos Post_, Mar 24 2012