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Primes of the form identical digits preceded by a 9.
1

%I #13 Jun 28 2020 12:32:40

%S 97,911,977,97777,911111,97777777777777777777,911111111111111111111,

%T 97777777777777777777777777777,

%U 911111111111111111111111111111111111111111

%N Primes of the form identical digits preceded by a 9.

%C The identical digits must be either 1 or 7. - _Harvey P. Dale_, Jun 28 2020

%H Vincenzo Librandi, <a href="/A090157/b090157.txt">Table of n, a(n) for n = 1..21</a>

%t Select[ FromDigits /@ Flatten[ Table[ PadRight[{9}, i, # ] & /@ {1, 2, 3, 4, 5, 6, 7, 8, 9}, {i, 2, 50}], 1], PrimeQ[ # ] &]

%t Select[Union[Flatten[Table[FromDigits[PadRight[{9},n,d]],{n,2,50},{d,{1,7}}]]],PrimeQ] (* _Harvey P. Dale_, Jun 27 2020 *)

%Y Cf. A090148, A090149, A090150, A090151, A090152, A090153, A090154, A090155, A090157.

%K nonn,base

%O 1,1

%A _Robert G. Wilson v_, Nov 22 2003