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 A185191 #16 Mar 29 2025 18:55:43
%S A185191 2,3,5,3,5,15,15,9,5,19,5,13,15,49,17,9,11,19,5,19,17,21,17,49,27,79,
%T A185191 89,33,41,19,17,25,77,49,17,31,87,19,167,31,17,67,117,69,57,127,65,
%U A185191 111,35,139,143,145,53,67,27,25,57,99,107,31,87,165,83
%N A185191 a(n) = 2^n - second largest prime less than 2^n.
%D A185191 D. E. Knuth, The Art of Computer Programming Second Edition. Vol. 2, Seminumerical Algorithms. Chapter 4.5.4 Factoring into Primes, Table 1, Page 390, Addison-Wesley, Reading, MA, 1981.
%H A185191 G. C. Greubel, <a href="/A185191/b185191.txt">Table of n, a(n) for n = 2..1000</a>
%F A185191 a(n) = 2^n - precprime(precprime(2^n)-1)
%e A185191 a(2)=2 because precprime(4)=3, and precprime(2)=2.
%t A185191 Table[2^n - NextPrime[2^n, -2], {n, 2, 64}] (* _T. D. Noe_, Jan 24 2012 *)
%t A185191 #-NextPrime[#,-2]&/@(2^Range[2,70]) (* _Harvey P. Dale_, Mar 29 2025 *)
%Y A185191 Cf. A013603.
%K A185191 nonn
%O A185191 2,1
%A A185191 _Washington Bomfim_, Jan 23 2012