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%I #38 Jun 25 2021 23:38:05
%S 1,12,123,1234,12345,123456,1234568,12345689,1234568910,123456891011,
%T 12345689101112,1234568910111213,123456891011121314,
%U 12345689101112131415,1234568910111213141516,123456891011121314151617,12345689101112131415161718,1234568910111213141516171819,123456891011121314151617181920
%N Concatenation of the numbers from 1 to n but omitting 7.
%C There are only three primes among the first 98 terms: 123456891011,
%C 12345689101112131415161718192021222324252627282930313233343536373839, and
%C 12345689101112131415161718192021222324252627282930313233343536373839\
%C 40414243444546474849505152535455565758596061626364566676869707172737\
%C 475767778798081828384858687.
%C Does the sequence contain any other primes?
%C Yes, PARI reports the next (probable) prime after appending 13097; it has 54655 digits. - _Bill McEachen_, Oct 13 2015
%F (a(n) - a(n-1)) mod 2 = 0 only for n=7. - _Altug Alkan_, Oct 15 2015
%t DeleteDuplicates@ Table[If[n >= 7, FromDigits@ Delete[Flatten[IntegerDigits /@ Range[n + 1]], 7], FromDigits@ Flatten[IntegerDigits /@ Range@ n]], {n, 19}] (* _Michael De Vlieger_, Oct 13 2015 *)
%Y Cf. A007908, A262299, A262571-A262582.
%Y See A262300 for more about this problem.
%K nonn,base
%O 1,2
%A _N. J. A. Sloane_, Sep 25 2015