OFFSET
1,1
COMMENTS
The numbers not of the form x^2+3y^2+3z^2.
Numbers whose squarefree part is congruent to 2 modulo 3. - Peter Munn, May 17 2020
The asymptotic density of this sequence is 3/8. - Amiram Eldar, Mar 08 2021
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
L. J. Mordell, A new Waring's problem with squares of linear forms, Quart. J. Math., 1 (1930), 276-288 (see p. 283).
FORMULA
a(n) = A055040(n)/3. - Peter Munn, May 17 2020
MATHEMATICA
max = 200; Select[ Union[ Flatten[ Table[ 9^i*(3*j + 2), {i, 0, Ceiling[Log[max]/Log[9]]}, {j, 0, Ceiling[( max/9^i - 2)/3]}]]], # <= max &] (* Jean-François Alcover, Oct 13 2011 *)
PROG
(Haskell)
a055048 n = a055048_list !! (n-1)
a055048_list = filter (s 0) [1..] where
s t u | m > 0 = even t && m == 2
| m == 0 = s (t + 1) u' where (u', m) = divMod u 3
-- Reinhard Zumkeller, Apr 07 2012
(PARI) is(n)=n/=9^valuation(n, 9); n%3==2 \\ Charles R Greathouse IV and V. Raman, Dec 19 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 01 2000
STATUS
approved