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%I #10 Oct 12 2017 07:57:39
%S 0,0,123,1021,10021,100021,1000021,10000021,100000021,1000000021,
%T 10000000021,100000000021,1000000000021,10000000000021,
%U 100000000000021,1000000000000021,10000000000000021,100000000000000021,1000000000000000021,10000000000000000021,100000000000000000021,1000000000000000000021
%N Smallest n-digit squarefree number whose internal as well as external digits form a squarefree number greater than 1; or 0 if no such number exists.
%C Note that the pattern 10^(n-1)+21 is broken for any n == 3 (mod 22), where 10^(n-1)+21 is divisible by 11^2. - _Franklin T. Adams-Watters_, Jun 13 2006
%e a(5) = 10021 = 11 * 911 is squarefree, internal digits 002 squarefree, external digits 11 squarefree.
%e a(6) = 100021 = 29 * 3449 has exactly those same properties.
%e a(7) = 1000021 = 11 * 90911 has exactly those same properties.
%e a(8) = 10000021 = 97 * 103093 has exactly those same properties.
%e a(9) = 100000021 = 11 * 19 * 23 * 71 * 293 is similar, though not semiprime.
%Y Cf. A077376, A077377, A077378, A077380.
%K base,nonn
%O 1,3
%A _Amarnath Murthy_, Nov 06 2002
%E a(5)-a(12) from _Jonathan Vos Post_, Mar 22 2006
%E More terms from _Franklin T. Adams-Watters_, Jun 13 2006