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A108842 Palindromic primes in which all internal digits are 5.

%I A108842 #25 Jan 28 2023 02:32:08
%S A108842 151,353,757,15551,75557,355555553,75555555557,155555555555555555551,
%T A108842 755555555555555555557,75555555555555555555557,
%U A108842 155555555555555555555555555555551,75555555555555555555555555555555555555555555555555555555557
%N A108842 Palindromic primes in which all internal digits are 5.
%C A108842 The next term -- a(13) -- has 75 digits. - _Harvey P. Dale_, May 18 2015
%C A108842 a(25) has 1975 digits. - _Michael S. Branicky_, Jan 27 2023
%H A108842 Michael S. Branicky, <a href="/A108842/b108842.txt">Table of n, a(n) for n = 1..24</a>
%t A108842 Select[Sort[Flatten[Table[FromDigits[Join[{n},PadRight[{},i,5],{n}]],{n,{1,3,7,9}},{i,80}]]],PrimeQ] (* _Harvey P. Dale_, May 18 2015 *)
%o A108842 (PARI) n10np1(n,d) = { local(x,y,k); for(x=1,n, for(k=1,8, y=10^(x+1)*k+floor(10^x*d/9)*10+k; if(isprime(y),print1(y",")) ) ) }
%o A108842 (Python)
%o A108842 from sympy import isprime
%o A108842 from itertools import count, islice
%o A108842 def agen(): yield from (t for i in count(1) for f in "1379" if isprime(t:=int(f + "5"*i + f)))
%o A108842 print(list(islice(agen(), 10))) # _Michael S. Branicky_, Jan 27 2023
%Y A108842 Similar sequences for digit d: A108845 (d=1), A108846 (d=2), A108841 (d=4), this sequence (d=5), A108843 (d=6), A108844 (d=7), A108847 (d=8).
%K A108842 easy,nonn,base
%O A108842 1,1
%A A108842 _Cino Hilliard_, Jul 11 2005
%E A108842 Name changed by _Arkadiusz Wesolowski_, Sep 07 2011
%E A108842 More terms from _Harvey P. Dale_, May 18 2015