A108844 - OEIS

%I #19 Jan 31 2023 08:29:31

%S 373,1777771,377777777777773,

%T 1777777777777777777777777777777777777777777777771,

%U 3777777777777777777777777777777777777777777777777777773,377777777777777777777777777777777777777777777777777777777777777777773

%N Palindromic primes in which all internal digits are 7.

%C a(13) has 1003 digits. - _Michael S. Branicky_, Jan 27 2023

%H Michael S. Branicky, <a href="/A108844/b108844.txt">Table of n, a(n) for n = 1..12</a>

%t 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 (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 (Python)

%o from sympy import isprime

%o from itertools import count, islice

%o def agen(): yield from (t for i in count(1) for f in "1379" if isprime(t:=int(f + "7"*i + f)))

%o print(list(islice(agen(), 12))) # _Michael S. Branicky_, Jan 27 2023

%Y Similar sequences for digit d: A108845 (d=1), A108846 (d=2), A108841 (d=4), A108842 (d=5), A108843 (d=6), A108847 (d=8).

%K easy,nonn,base

%O 1,1

%A _Cino Hilliard_, Jul 11 2005

%E Name changed by _Arkadiusz Wesolowski_, Sep 07 2011

%E One more term (a(6)) added by _Harvey P. Dale_, Nov 22 2012