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Primes of the form identical digits preceded by a 5.
2

%I #7 May 31 2020 12:48:10

%S 5,53,59,577,599,5333,59999,511111,599999,59999999,577777777,

%T 59999999999,5111111111111,53333333333333,59999999999999,

%U 577777777777777,5111111111111111,577777777777777777

%N Primes of the form identical digits preceded by a 5.

%H Vincenzo Librandi, <a href="/A090153/b090153.txt">Table of n, a(n) for n = 1..43</a>

%t Select[ FromDigits /@ Flatten[ Table[ PadRight[{5}, i, # ] & /@ {1, 2, 3, 4, 5, 6, 7, 8, 9}, {i, 2, 24}], 1], PrimeQ[ # ] &]

%t Join[{5},Select[Flatten[Table[FromDigits[PadRight[{5},n,k]],{n,2,20},{k,{1,3,7,9}}]],PrimeQ]] (* _Harvey P. Dale_, May 31 2020 *)

%K nonn,base

%O 1,1

%A _Robert G. Wilson v_, Nov 22 2003