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A210665 Least semiprime dividing digit reversal of n-th semiprime, or 0 if no such factor.

%I A210665 #16 Feb 28 2015 23:16:20
%S A210665 4,6,9,0,0,51,4,22,4,62,33,0,0,0,93,4,94,15,55,15,85,26,4,4,0,77,4,58,
%T A210665 4,6,0,39,49,0,0,111,511,0,0,121,221,321,921,0,0,141,0,341,0,0,551,
%U A210665 851,951,161,0,961,771,871,381,581,781,0,6,202,302,502,14
%N A210665 Least semiprime dividing digit reversal of n-th semiprime, or 0 if no such factor.
%H A210665 Alois P. Heinz, <a href="/A210665/b210665.txt">Table of n, a(n) for n = 1..10000</a>
%F A210665 a(n) = A210615(A210616(n)).
%e A210665 a(4) = 0 because the 4th semiprime is 10, and R(10) = 1, which is not divisible by any semiprime.
%e A210665 a(6) = 51 because the 6th semiprime is 15, and R(15) = 51, which is itself semiprime.
%e A210665 a(7) = 4 because the 7th semiprime is 21, R(21) = 12, and 4 is the least semiprime divisor of 12.
%p A210665 r:= proc(n) option remember; local q;
%p A210665       `if`(n<10, n, irem(n, 10, 'q') *10^(length(n)-1)+r(q))
%p A210665     end:
%p A210665 b:= proc(n) option remember; local k;
%p A210665       if n=0 then 0
%p A210665     else for k from b(n-1)+1
%p A210665            while isprime(k) or 2<>add (i[2], i=ifactors(k)[2])
%p A210665          do od; k
%p A210665       fi
%p A210665     end:
%p A210665 a:= proc(n) option remember; local m, k;
%p A210665       m:= r(b(n));
%p A210665       for k from 4 to m do
%p A210665          if irem(m, k)=0 and not isprime(k) and
%p A210665             add(i[2], i=ifactors(k)[2])=2 then return k fi
%p A210665       od; 0
%p A210665     end:
%p A210665 seq(a(n), n=1..100);  # _Alois P. Heinz_, Mar 28 2012
%Y A210665 Cf. A001358, A210615, A210616.
%K A210665 nonn,base,look,easy
%O A210665 1,1
%A A210665 _Jonathan Vos Post_, Mar 28 2012