A068531 a(n) = (3^(2^n) - 1)/2^(n + 2).

1, 5, 205, 672605, 14476720225405, 13412827423017626893194723005, 23027704253395670256876704807446325518902757016163752166205, 135750441774555403090761510536778616322479346492704236319926586357457102177506285098634540189560165548644204629442284605

Offset: 1

Author

Benoit Cloitre, Mar 22 2002

Keywords

• easy
• nonn

Comments

Every element of this sequence is an odd number (see link). - Graeme McRae, Jan 12 2005

Links

• Graeme McRae, Proof: for every positive integer k, there exists a positive integer m such that 3^m+5 is divisible by 2^k.

Crossrefs

Cf. A090129.

Programs

Mathematica

a[n_] := (3^(2^n) - 1)/2^(n + 2); Array[a, 8] (* Amiram Eldar, May 07 2025 *)