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%I #16 Jun 15 2022 12:53:01
%S 17,1777,1777777777,17777777777777,
%T 1777777777777777777777777777777777777777777,
%U 1777777777777777777777777777777777777777777777777777
%N Primes whose leftmost decimal digit is 1 and whose other decimal digits are all 7's.
%C In no element of this sequence can the number of 7's be congruent to 2 modulo 3.
%H Harvey P. Dale, <a href="/A088465/b088465.txt">Table of n, a(n) for n = 1..12</a>
%H Vladislav Avrutin, <a href="http://primepyramids.by.ru/">Prime pyramids</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/17777.htm#prime">Prime numbers of the form 177...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%t Do[ a = 10^n + 7(10^n - 1)/9; If[ PrimeQ[a], Print[a]], {n, 1, 53}] (* _Robert G. Wilson v_, Dec 05 2003 *)
%t Select[Table[FromDigits[PadRight[{1},n,7]],{n,0,60}],PrimeQ] (* _Harvey P. Dale_, Jun 15 2022 *)
%Y Cf. A002275, A089147 (number of sevens).
%K base,easy,nonn
%O 1,1
%A _Sam Alexander_, Nov 12 2003
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