

A001650


n appears n times (n odd).


10



1, 3, 3, 3, 5, 5, 5, 5, 5, 7, 7, 7, 7, 7, 7, 7, 9, 9, 9, 9, 9, 9, 9, 9, 9, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17
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OFFSET

1,2


COMMENTS

For n >= 0, a(n+1) is the number of integers x with x <= sqrt(n), or equivalently the number of point in the Z^1 lattice of norm <= n+1.  David W. Wilson, Oct 22 2006


REFERENCES

J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", SpringerVerlag, p. 106.


LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000
Abraham Isgur, Vitaly Kuznetsov, and Stephen Tanny, A combinatorial approach for solving certain nested recursions with nonslow solutions, arXiv preprint arXiv:1202.0276, 2012


FORMULA

a(n) = 1 + 2*floor(sqrt(n1)), n > 0.  Antonio Esposito (antonio.b.esposito(AT)italtel.it), Jan 21 2002
G.f.: theta_3(x)*x/(1x). a(n+1)=a(n)+A000122(n).  Michael Somos, Apr 29 2003.
a(1)=1,a(2)=3,a(3)=3,a(n)=a(na(n2))+2.  Branko Curgus, May 07 2010
a(n)=2*ceiling(sqrt(n))1.  Branko Curgus, May 07 2010
Seen as a triangle read by rows: T(n,k) = 2*(n1), k=1..n.  Reinhard Zumkeller, Nov 14 2015


MATHEMATICA

a[1]=1, a[2]=3, a[3]=3, a[n_]:=a[n]=a[na[n2]]+2 (* Branko Curgus, May 07 2010 *)
Flatten[Table[Table[n, {n}], {n, 1, 17, 2}]] (* Harvey P. Dale, Mar 31 2013 *)


PROG

(PARI) a(n)=if(n<1, 0, 1+2*sqrtint(n1))
(Haskell)
a001650 n k = a001650_tabf !! (n1) !! (k1)
a001650_row n = a001650_tabf !! (n1)
a001650_tabf = iterate (\xs@(x:_) > map (+ 2) (x:x:xs)) [1]
a001650_list = concat a001650_tabf
 Reinhard Zumkeller, Nov 14 2015


CROSSREFS

Cf. A001670. Partial sums of A000122.
Cf. A111650, A131507, A193832.
Sequence in context: A136800 A126661 A162226 * A130175 A200266 A101290
Adjacent sequences: A001647 A001648 A001649 * A001651 A001652 A001653


KEYWORD

nonn,easy


AUTHOR

N. J. A. Sloane.


EXTENSIONS

More terms from Michael Somos, Apr 29 2003.


STATUS

approved



