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 A013604 #14 Sep 08 2022 08:44:38
%S A013604 1,2,4,2,2,2,8,8,2,20,16,58,22,8,16,98,10,10,14,8,4,2,20,28,20,20,26,
%T A013604 22,14,26,4,34,52,56,28,10,2,10,58,38,2,122,74,34,22,22,46,44,50,46,
%U A013604 16,124,4,10,106,10,26,220,38,160,74,8,28,104,104,38,298,94,16
%N A013604 a(n) = 3^n - prevprime(3^n), where prevprime(x) is the largest prime < x.
%H A013604 Zak Seidov, <a href="/A013604/b013604.txt">Table of n, a(n) for n = 1..1000</a>
%F A013604 a(n) = A049711(3^n) = A049711(A000244(n)). - _Michel Marcus_, Sep 16 2016
%p A013604 seq(3^i-prevprime(3^i),i=1..100);
%t A013604 Table[3^i - NextPrime[3^i, - 1], {i, 70}] (* _Vincenzo Librandi_, Sep 16 2016 *)
%o A013604 (Magma) [3^n-PreviousPrime(3^n): n in [1..70]]; // _Vincenzo Librandi_, Sep 16 2016
%o A013604 (PARI) a(n) = 3^n - precprime(3^n-1); \\ _Michel Marcus_, Sep 16 2016
%Y A013604 Cf. A000244, A013598, A049711.
%K A013604 nonn
%O A013604 1,2
%A A013604 James Kilfiger (mapdn(AT)csv.warwick.ac.uk)