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A000164 Number of partitions of n into 3 squares (allowing part zero). 7
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 (list; graph; refs; listen; history; text; internal format)
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.

Hirschhorn, M. D.; Some formulas for partitions into squares, DiscreteMath, 211 (2000), pp. 225-228. [From Ant King, Oct 15 2010]

LINKS

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

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)=5delta(n)+3 delta(1/2 n)+ 4delta(1/3 n), beta(n)=4e(n,1,3,4)+3e(n,1,7,8)+3e(n,3,5,8), gamma(n)=2 sum(e(n-k^2,1,3,4),1<=k^2<n), 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 + ...

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 */

CROSSREFS

Sequence in context: A267611 A178666 A206706 * A157746 A037820 A076493

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

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Last modified December 8 20:44 EST 2016. Contains 278950 sequences.