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%I #14 Jan 28 2023 12:11:07
%S 727,757,787,797,72227,75557,76667,78887,79997,7666667,722222227,
%T 74444444447,75555555557,755555555555555555557,
%U 75555555555555555555557,72222222222222222222222222227,79999999999999999999999999997,7444444444444444444444444444447
%N Primes which are a sandwich of numbers made of only one digit between two 7's.
%C The middle digit is never 0, 1, 3, or 7. - _Harvey P. Dale_, May 05 2018
%C a(40) has 1213 digits. - _Michael S. Branicky_, Jan 28 2023
%H Harvey P. Dale, <a href="/A068688/b068688.txt">Table of n, a(n) for n = 1..39</a>
%t Select[Flatten[Table[10FromDigits[PadRight[{7},n,i]]+7,{n,2,100},{i,9}]],PrimeQ] (* _Harvey P. Dale_, May 05 2018 *)
%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 m in "0123456789" if isprime(t:=int("7" + m*i + "7")))
%o print(list(islice(agen(), 30))) # _Michael S. Branicky_, Jan 28 2023
%Y Cf. A068685, A068687, A068689.
%K base,nonn
%O 1,1
%A _Amarnath Murthy_, Mar 02 2002
%E More terms from _Sascha Kurz_, Mar 17 2002
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