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A112750
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Smallest prime of the form 7 followed by j copies of the digit k, where j runs through those positive values for which such a prime exists.
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0
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71, 733, 7333, 79999, 733333, 71111111, 799999999, 79999999999, 79999999999999999999999999, 79999999999999999999999999999999999999999999999999, 733333333333333333333333333333333333333333333333333333, 71111111111111111111111111111111111111111111111111111111
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OFFSET
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1,1
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COMMENTS
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For all j > 0, k must be 1, 3, or 9, since a number with --
-- digits 7kk...kk where k is even will be a proper multiple of 2;
-- digits 755...55 will be a proper multiple of 5; and
-- digits 777...77 will be a proper multiple of 7.
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LINKS
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EXAMPLE
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7333 is a term because it is prime and is 7 followed by three copies of 3, and the numbers 7000, 7111, and 7222 are all nonprime.
Terms begin as follows:
n j k a(n)
-- -- - --------------------------------------------------------
1 1 1 71
2 2 3 733
3 3 3 7333
4 4 9 79999
5 5 3 733333
- 6 - (7111111, 7333333, 7999999 are composite)
6 7 1 71111111
7 8 9 799999999
- 9 - (7111111111, 7333333333, 7999999999 are composite)
8 10 9 79999999999
- 11 - (711111111111, 733333333333, 799999999999 are composite)
- 12 - (all composite)
- 13 - (all composite)
...
9 25 9 79999999999999999999999999
...
10 49 9 79999999999999999999999999999999999999999999999999
...
11 53 3 733333333333333333333333333333333333333333333333333333
12 55 1 71111111111111111111111111111111111111111111111111111111
(End)
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MATHEMATICA
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SelectFirst[#, PrimeQ]&/@Table[FromDigits[PadRight[{7}, n, p]], {n, 2, 60}, {p, {1, 3, 9}}]/.Missing["NotFound"]->Nothing (* Harvey P. Dale, Apr 19 2021 *)
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CROSSREFS
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KEYWORD
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base,nonn,less
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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