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 A068531 #33 May 08 2025 02:29:07 %S A068531 1,5,205,672605,14476720225405,13412827423017626893194723005, %T A068531 23027704253395670256876704807446325518902757016163752166205, %U A068531 135750441774555403090761510536778616322479346492704236319926586357457102177506285098634540189560165548644204629442284605 %N A068531 a(n) = (3^(2^n) - 1)/2^(n + 2). %C A068531 Every element of this sequence is an odd number (see link). - _Graeme McRae_, Jan 12 2005 %H A068531 Graeme McRae, <a href="https://web.archive.org/web/20150318204102/http://2000clicks.com/mathhelp/PuzzlePowersOf3AndPowersOf2Answer.aspx">Proof: for every positive integer k, there exists a positive integer m such that 3^m+5 is divisible by 2^k</a>. %t A068531 a[n_] := (3^(2^n) - 1)/2^(n + 2); Array[a, 8] (* _Amiram Eldar_, May 07 2025 *) %Y A068531 Cf. A090129. %K A068531 easy,nonn %O A068531 1,2 %A A068531 _Benoit Cloitre_, Mar 22 2002