This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A077379 #10 Oct 12 2017 07:57:39 %S A077379 0,0,123,1021,10021,100021,1000021,10000021,100000021,1000000021, %T A077379 10000000021,100000000021,1000000000021,10000000000021, %U A077379 100000000000021,1000000000000021,10000000000000021,100000000000000021,1000000000000000021,10000000000000000021,100000000000000000021,1000000000000000000021 %N A077379 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 A077379 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 A077379 a(5) = 10021 = 11 * 911 is squarefree, internal digits 002 squarefree, external digits 11 squarefree. %e A077379 a(6) = 100021 = 29 * 3449 has exactly those same properties. %e A077379 a(7) = 1000021 = 11 * 90911 has exactly those same properties. %e A077379 a(8) = 10000021 = 97 * 103093 has exactly those same properties. %e A077379 a(9) = 100000021 = 11 * 19 * 23 * 71 * 293 is similar, though not semiprime. %Y A077379 Cf. A077376, A077377, A077378, A077380. %K A077379 base,nonn %O A077379 1,3 %A A077379 _Amarnath Murthy_, Nov 06 2002 %E A077379 a(5)-a(12) from _Jonathan Vos Post_, Mar 22 2006 %E A077379 More terms from _Franklin T. Adams-Watters_, Jun 13 2006