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A108844 Palindromic primes in which all internal digits are 7.

%I A108844 #19 Jan 31 2023 08:29:31
%S A108844 373,1777771,377777777777773,
%T A108844 1777777777777777777777777777777777777777777777771,
%U A108844 3777777777777777777777777777777777777777777777777777773,377777777777777777777777777777777777777777777777777777777777777777773
%N A108844 Palindromic primes in which all internal digits are 7.
%C A108844 a(13) has 1003 digits. - _Michael S. Branicky_, Jan 27 2023
%H A108844 Michael S. Branicky, <a href="/A108844/b108844.txt">Table of n, a(n) for n = 1..12</a>
%t A108844 Union[Flatten[Table[Select[Table[FromDigits[Join[{i},PadRight[{},n,7],{i}]],{n,100}],PrimeQ],{i,1,9,2}]]] (* _Harvey P. Dale_, Nov 22 2012 *)
%o A108844 (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 A108844 (Python)
%o A108844 from sympy import isprime
%o A108844 from itertools import count, islice
%o A108844 def agen(): yield from (t for i in count(1) for f in "1379" if isprime(t:=int(f + "7"*i + f)))
%o A108844 print(list(islice(agen(), 12))) # _Michael S. Branicky_, Jan 27 2023
%Y A108844 Similar sequences for digit d: A108845 (d=1), A108846 (d=2), A108841 (d=4), A108842 (d=5), A108843 (d=6), A108847 (d=8).
%K A108844 easy,nonn,base
%O A108844 1,1
%A A108844 _Cino Hilliard_, Jul 11 2005
%E A108844 Name changed by _Arkadiusz Wesolowski_, Sep 07 2011
%E A108844 One more term (a(6)) added by _Harvey P. Dale_, Nov 22 2012