A168493 a(n) = 3^floor(n^2/2).

1, 1, 9, 81, 6561, 531441, 387420489, 282429536481, 1853020188851841, 12157665459056928801, 717897987691852588770249, 42391158275216203514294433201, 22528399544939174411840147874772641, 11972515182562019788602740026717047105681

Offset: 0

Author

Paul Barry, Nov 27 2009

Keywords

• easy
• nonn

Comments

Hankel transform of A168492. A trivial Somos-4 sequence linked to elliptic curve y^2 = 1 - 16x + 76x^2 - 96x^3.

Links

• G. C. Greubel, Table of n, a(n) for n = 0..50

Programs

Magma

[3^Floor(n^2/2): n in [0..15]]; // Vincenzo Librandi, Jul 24 2016

Mathematica

Table[3^Floor[n^2/2],{n,0,20}]  (* Harvey P. Dale, Mar 06 2011 *)

Formula

a(n) = 3^floor(n^2/2) = 9^floor(n^2/4) = 9^A002620(n) = 81*a(n-2)^2/a(n-4).