A213681 a(n) = a(n-1)^2/2 - 4 with a(0) = 6.

6, 14, 94, 4414, 9741694, 47450300994814, 1125765532249223239027447294, 633674016800188444301553476967536472012841942973961214

Offset

0,1

Comments

The next term has 108 digits. - Harvey P. Dale, Apr 21 2018

Links

Table of n, a(n) for n=0..7.

Formula

a(n) = 4*cosh(2^n*arccosh(3/2)).

a(n) = 2*A005248(2^n). [Bruno Berselli, Mar 04 2013]

Example

a(2) = a(1)^2/2-4 = 14^2/2-4 = 94.

Mathematica

NestList[#^2/2-4&, 6, 8] (* Harvey P. Dale, Apr 21 2018 *)

Prog

(Sage)
var('x')
p=x^2/2-4
s=[6]
for i in [0..8]:
    s=s+[p(s[i])]
show(s)

Crossrefs

Cf. A005248.

Sequence in context: A200033 A219376 A130263 * A308489 A301425 A077401

Adjacent sequences: A213678 A213679 A213680 * A213682 A213683 A213684

Keyword

nonn

Author

Jose Eduardo Blazek, Mar 04 2013

Status

approved