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 A014234 #50 Jun 03 2024 16:04:55
%S A014234 2,3,7,13,31,61,127,251,509,1021,2039,4093,8191,16381,32749,65521,
%T A014234 131071,262139,524287,1048573,2097143,4194301,8388593,16777213,
%U A014234 33554393,67108859,134217689,268435399,536870909,1073741789,2147483647,4294967291,8589934583,17179869143,34359738337,68719476731,137438953447
%N A014234 Largest prime <= 2^n.
%C A014234 For n>1 largest prime factor of the denominator of A027611(2^n) = 2^n*(2^n)-th harmonic number. - _Alexander Adamchuk_, Aug 02 2006
%D A014234 D. E. Knuth, The Art of Computer Programming. Addison-Wesley, Reading, MA, Vol. 2, p. 390.
%H A014234 T. D. Noe, <a href="/A014234/b014234.txt">Table of n, a(n) for n=1..1000</a>
%H A014234 Fred Curtis, <a href="/A014234/a014234_1.txt">C++ program for A014234</a>
%H A014234 Harry J. Smith, <a href="http://www.math.niu.edu/~rusin/known-math/98/pi_x">PrimePi2 - Computes the Prime Pi(x) counting function</a> [Broken link]
%H A014234 Harry J. Smith, <a href="/A014234/a014234.txt">PrimePi2 - Computes the Prime Pi(x) counting function</a> [Cached copy]
%p A014234 a:= n-> prevprime(2^n+1):
%p A014234 seq(a(n), n=1..40); # _Alois P. Heinz_, Apr 23 2020
%t A014234 PrevPrim[n_] := Block[{k = n - 1}, While[ !PrimeQ[k], k-- ]; k]; Table[ Abs[ PrevPrim[2^n]], {n, 1, 30} ]
%t A014234 Join[{2},NextPrime[2^Range[2,40],-1]] (* _Harvey P. Dale_, Jun 26 2011 *)
%o A014234 (C++) // see link above
%o A014234 (PARI) a(n) = precprime(2^n) \\ _Michel Marcus_, Aug 08 2013
%Y A014234 Cf. A000079, A014210.
%Y A014234 Cf. A013603 (2^n - a(n)).
%Y A014234 See comment for the relationship to A027611.
%Y A014234 These primes have indices A007053 = number of primes <= 2^n.
%Y A014234 The opposite is A104080, delta A092131, indices A372684.
%Y A014234 For squarefree instead of prime we have A372889, indices A143658.
%Y A014234 A036378 counts primes between powers of 2, A293697 adds them up.
%Y A014234 Cf. A035100, A130739, A211997.
%K A014234 nonn
%O A014234 1,1
%A A014234 _Jud McCranie_
%E A014234 Terms for n=31, n=32 added by Fred Curtis (fred(AT)f2.org), Dec 08 2009