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A006456
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Number of compositions (ordered partitions) of n into squares.
(Formerly M0528)
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56
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1, 1, 1, 1, 2, 3, 4, 5, 7, 11, 16, 22, 30, 43, 62, 88, 124, 175, 249, 354, 502, 710, 1006, 1427, 2024, 2870, 4068, 5767, 8176, 11593, 16436, 23301, 33033, 46832, 66398, 94137, 133462, 189211, 268252, 380315, 539192, 764433, 1083764, 1536498, 2178364
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OFFSET
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0,5
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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Jan Bohman, Carl-Erik Fröberg, Hans Riesel, Partitions in squares, Nordisk Tidskr. Informationsbehandling (BIT) 19 (1979), 297-301.
J. Bohman et al., Partitions in squares, Nordisk Tidskr. Informationsbehandling (BIT) 19 (1979), 297-301. (Annotated scanned copy)
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FORMULA
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a(n) = 1, if n = 0; a(n)=Sum(1 <= k^2 <= n, a(n-k^2)), if n > 0. - David W. Wilson
G.f.: 1/(1-x-x^4-x^9-....) - Jon Perry, Jul 04 2004
a(n) ~ c * d^n, where d is the root of the equation EllipticTheta(3, 0, 1/d) = 3, d = 1.41774254618138831428829091099000662953179532057717725688..., c = 0.46542113389379672452973940263069782869244877335179331541... - Vaclav Kotesovec, May 01 2014, updated Jan 05 2017
G.f.: 2/(3 - theta_3(q)), where theta_3() is the Jacobi theta function. - Ilya Gutkovskiy, Aug 08 2018
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MATHEMATICA
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a[n_]:=a[n]=If[n==0, 1, Sum[a[n - k], {k, Select[Range[n], IntegerQ[Sqrt[#]] &]}]]; Table[a[n], {n, 0, 100}] (* Indranil Ghosh, Jul 28 2017, after David W. Wilson's formula *)
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PROG
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(PARI)
N=66; x='x+O('x^N);
Vec( 1/( 1 - sum(k=1, 1+sqrtint(N), x^(k^2) ) ) )
(Python)
from gmpy2 import is_square
class Memoize:
def __init__(self, func):
self.func=func
self.cache={}
def __call__(self, arg):
if arg not in self.cache:
self.cache[arg] = self.func(arg)
return self.cache[arg]
@Memoize
def a(n): return 1 if n==0 else sum([a(n - k) for k in range(1, n + 1) if is_square(k)])
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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