A000164 Number of partitions of n into 3 squares (allowing part zero).

1, 1, 1, 1, 1, 1, 1, 0, 1, 2, 1, 1, 1, 1, 1, 0, 1, 2, 2, 1, 1, 1, 1, 0, 1, 2, 2, 2, 0, 2, 1, 0, 1, 2, 2, 1, 2, 1, 2, 0, 1, 3, 1, 1, 1, 2, 1, 0, 1, 2, 3, 2, 1, 2, 3, 0, 1, 2, 1, 2, 0, 2, 2, 0, 1, 3, 3, 1, 2, 2, 1, 0, 2, 2, 3, 2, 1, 2, 1, 0, 1, 4, 2, 2, 1, 2, 3, 0, 1, 4, 3, 1, 0, 1, 2, 0, 1, 2, 3, 3, 2, 4, 2, 0, 2

Offset

0,10

Comments

a(n) = number of triples of integers [ i, j, k] such that i >= j >= k >= 0 and n = i^2 + j^2 + k^2. - Michael Somos, Jun 05 2012

References

E. Grosswald, Representations of Integers as Sums of Squares. Springer-Verlag, NY, 1985, p. 84.

Links

T. D. Noe, Table of n, a(n) for n = 0..10000

Hirschhorn, M. D., Some formulas for partitions into squares, Discrete Math. 211 (2000), pp. 225-228.

Formula

Let e(n,r,s,m) be the excess of the number of n's r(mod m) divisors over the number of its s (mod m) divisors, and let delta(n)=1 if n is a perfect square and 0 otherwise. Then, if we define alpha(n) = 5*delta(n) + 3*delta(n/2) + 4*delta(n/3), beta(n) = 4*e(n,1,3,4) + 3*e(n,1,7,8) + 3*e(n,3,5,8), gamma(n) = 2*Sum_{1<=k^2<n} e(n-k^2,1,3,4), it follows that a(n) = (1/12)*(alpha(n) + beta(n) + gamma(n)). - Ant King, Oct 15 2010

Example

G.f. = 1 + x + x^2 + x^3 + x^4 + x^5 + x^6 + x^8 + 2*x^9 + x^10 + x^11 + x^12 + x^13 + ...

Maple

A000164 := proc(n)
    local a, x, y, z2, z ;
    a := 0 ;
    for x from 0 do
        if 3*x^2 > n then
            return a;
        end if;
        for y from x do
            if x^2+2*y^2 > n then
                break;
            end if;
            z2 := n-x^2-y^2 ;
            if issqr(z2) then
                z := sqrt(z2) ;
                if z >= y then
                    a := a+1 ;
                end if;
            end if;
        end do:
    end do:
    a;
end proc: # R. J. Mathar, Feb 12 2017

Mathematica

Length[PowersRepresentations[ #, 3, 2]] & /@ Range[0, 104]
e[0, r_, s_, m_]=0; e[n_, r_, s_, m_]:=Length[Select[Divisors[n], Mod[ #, m]==r &]]-Length[Select[Divisors[n], Mod[ #, m]==s &]]; alpha[n_]:=5delta[n]+3delta[1/2 n]+4delta[1/3n]; beta[n_]:=4e[n, 1, 3, 4]+3e[n, 1, 7, 8]+3e[n, 3, 5, 8]; delta[n_]:=If[IntegerQ[Sqrt[n]], 1, 0]; f[n_]:=Table[n-k^2, {k, 1, Floor[Sqrt[n]]}]; gamma[n_]:=2 Plus@@(e[ #, 1, 3, 4] &/@f[n]); p3[n_]:=1/12(alpha[n]+beta[n]+gamma[n]); p3[ # ] &/@Range[0, 104]
(* Ant King, Oct 15 2010 *)
a[ n_] := If[ n < 0, 0, Sum[ Boole[ n == i^2 + j^2 + k^2], {i, 0, Sqrt[n]}, {j, 0, i}, {k, 0, j}]]; (* Michael Somos, Aug 15 2015 *)

Prog

(PARI) {a(n) = if( n<0, 0, sum( i=0, sqrtint(n), sum( j=0, i, sum( k=0, j, n == i^2 + j^2 + k^2))))}; /* Michael Somos, Jun 05 2012 */
(Python) import collections; a = collections.Counter(i*i + j*j + k*k for i in range(100) for j in range(i+1) for k in range(j+1)) # David Radcliffe, Apr 15 2019

Crossrefs

Equivalent sequences for other numbers of squares: A000161 (2), A002635 (4), A000174 (5).

Cf. A004215 (positions of zeros), A094942 (positions of ones), A124966 (positions of greater values).

Cf. A005875, A016727.

Sequence in context: A178666 A206706 A302354 * A330261 A157746 A349082

Adjacent sequences: A000161 A000162 A000163 * A000165 A000166 A000167

Keyword

nonn

Author

N. J. A. Sloane

Extensions

Name clarified by Wolfdieter Lang, Apr 08 2013

Status

approved