This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A083978 #15 Mar 25 2019 13:32:55
%S A083978 7,727,72727,77377,1777771,7772777,17772727771,777767777,72777777727,
%T A083978 77777677777,7767777777677,7777774777777,377777777777773,
%U A083978 777777727777777,72777777777777727,77777777677777777,7776777777777776777,717777777787777777717,737777777777777777737
%N A083978 Smallest palindromic prime containing exactly n 7's.
%H A083978 Chai Wah Wu, <a href="/A083978/b083978.txt">Table of n, a(n) for n = 1..500</a>
%t A083978 Table[d = 7; i = 1;
%t A083978 While[ x = IntegerDigits[Prime[i]];
%t A083978 x != Reverse[x] || Count[x, d] != n , i++];
%t A083978 FromDigits[x], {n, 1, 6}] (* _Robert Price_, Mar 25 2019 *)
%Y A083978 Cf. A083972, A083973, A083974, A083975, A083976, A083977, A083979, A083980, A083981.
%K A083978 base,nonn
%O A083978 1,1
%A A083978 _Amarnath Murthy_ and Meenakshi Srikanth (menakan_s(AT)yahoo.com), May 21 2003
%E A083978 Corrected and extended by _Ray G. Opao_, Sep 08 2004
%E A083978 Corrected and extended by _Giovanni Resta_, Feb 08 2006