A014980 a(n+1) = floor(a(n)/2) * ceiling(a(n)/2), a(0) = 5.

5, 6, 9, 20, 100, 2500, 1562500, 610351562500, 93132257461547851562500, 2168404344971008868014905601739883422851562500, 1175494350822287507968736537222245677818665556772087521508751706278417259454727172851562500

Offset: 0

Author

Colin Sandon (sandon(AT)together.net)

Keywords

• nonn
• nice
• easy

Comments

A194079(n) gives number of digits of a(n).

Examples

			6 = 2*3; 9 = 3*3; 20 = 4*5; ...
		

Crossrefs

Cf. A002620, A194079.

Programs

Haskell

a014980 n = a014980_list !! n
a014980_list = iterate a002620 5
-- Reinhard Zumkeller, Oct 12 2011

Mathematica

a=5;a=Table[a=Ceiling[a/2]*Floor[a/2],{n,0,10}] (* Vladimir Joseph Stephan Orlovsky, Apr 13 2010 *)
NestList[Floor[#/2]Ceiling[#/2]&,5,10] (* Harvey P. Dale, Jul 10 2012 *)

Python

from itertools import accumulate
def f(an, _): return (an//2)*((an+1)//2)
print(list(accumulate([5]*11, f))) # Michael S. Branicky, May 06 2021

Formula

a(0) = 5; a(k+1) = floor(a(k)/2) * ceiling(a(k)/2).

a(n+1) = A002620(a(n)), a(0) = 5. - Reinhard Zumkeller, Oct 12 2011

Extensions

More terms from James Sellers, Feb 05 2000