A090157
Primes of the form identical digits preceded by a 9.
Original entry on oeis.org
97, 911, 977, 97777, 911111, 97777777777777777777, 911111111111111111111, 97777777777777777777777777777, 911111111111111111111111111111111111111111
Offset: 1
-
Select[ FromDigits /@ Flatten[ Table[ PadRight[{9}, i, # ] & /@ {1, 2, 3, 4, 5, 6, 7, 8, 9}, {i, 2, 50}], 1], PrimeQ[ # ] &]
Select[Union[Flatten[Table[FromDigits[PadRight[{9},n,d]],{n,2,50},{d,{1,7}}]]],PrimeQ] (* Harvey P. Dale, Jun 27 2020 *)
A112750
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.
Original entry on oeis.org
71, 733, 7333, 79999, 733333, 71111111, 799999999, 79999999999, 79999999999999999999999999, 79999999999999999999999999999999999999999999999999, 733333333333333333333333333333333333333333333333333333, 71111111111111111111111111111111111111111111111111111111
Offset: 1
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.
From _Jon E. Schoenfield_, Feb 23 2021: (Start)
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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SelectFirst[#,PrimeQ]&/@Table[FromDigits[PadRight[{7},n,p]],{n,2,60},{p,{1,3,9}}]/.Missing["NotFound"]->Nothing (* Harvey P. Dale, Apr 19 2021 *)
Showing 1-2 of 2 results.
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