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A000161 Number of partitions of n into 2 squares. 46
1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 0, 1, 0, 0, 0, 0, 2, 1, 0, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 2, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 2, 0, 0, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 2, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 2, 1, 0, 0, 1, 0, 1, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,26

COMMENTS

Number of ways of writing n as a sum of 2 (possibly zero) squares when order does not matter.

Number of similar sublattices of square lattice with index n.

Let Pk = the number of partitions of n into k nonzero squares. Then we have A000161 = P0 + P1 + P2, A002635 = P0 + P1 + P2 + P3 + P4, A010052 = P1, A025426 = P2, A025427 = P3, A025428 = P4. - Charles R Greathouse IV, Mar 08 2010, amended by M. F. Hasler, Jan 25 2013

a(A022544(n))=0; a(A001481(n))>0; a(A125022(n))=1; a(A118882(n))>1. - Reinhard Zumkeller, Aug 16 2011

REFERENCES

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

J. V. Uspensky and M. A. Heaslet, Elementary Number Theory, McGraw-Hill, NY, 1939, p. 339

LINKS

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

H. Bottomley, Illustration of initial terms

R. T. Bumby, Sums of four squares, in Number theory (New York, 1991-1995), 1-8, Springer, New York, 1996.

J. H. Conway, E. M. Rains and N. J. A. Sloane, On the existence of similar sublattices, Canad. J. Math. 51 (1999), 1300-1306 (Abstract, pdf, ps).

Michael Gilleland, Some Self-Similar Integer Sequences

M. D. Hirschhorn, Some formulae for partitions into squares, Discrete Math, 211 (2000), pp. 225-228. [From Ant King, Oct 05 2010]

Index entries for sequences related to sublattices

Index entries for sequences related to sums of squares

Index entries for "core" sequences

FORMULA

a(n) = card { { a,b } c N | a^2+b^2 = n }. - M. F. Hasler, Nov 23 2007

Let f(n)= the number of divisors of n that are congruent to 1 modulo 4 minus the number of its divisors that are congruent to 3 modulo 4, and define delta(n) to be 1 if n is a perfect square and 0 otherwise. Then a(n)=1/2 (f(n)+delta(n)+delta(1/2 n)). - Ant King, Oct 05 2010

EXAMPLE

25 = 3^2+4^2 = 5^2, so a(25) = 2.

MAPLE

A000161 := proc(n) local i, j, ans; ans := 0; for i from 0 to n do for j from i to n do if i^2+j^2=n then ans := ans+1 fi od od; RETURN(ans); end; [ seq(A000161(i), i=0..50) ];

A000161 := n -> nops( numtheory[sum2sqr](n) ); # M. F. Hasler, Nov 23 2007

MATHEMATICA

Length[PowersRepresentations[ #, 2, 2]] &/@Range[0, 150] (* Ant King, Oct 05 2010 *)

PROG

(PARI) A000161(n)=sum(i=0, n, sum(j=0, i, if(i^2+j^2-n, 0, 1)))

(PARI) A000161(n)=sum(i=0, sqrtint(n>>1), issquare(n-i^2)) \\ M. F. Hasler, Nov 23 2007

(PARI) a(n)=sum(k=sqrtint((n-1)\2)+1, sqrtint(n), issquare(n-k^2)) \\ Charles R Greathouse IV, Mar 21 2014

(Haskell)

a000161 n =

   sum $ map (a010052 . (n -)) $ takeWhile (<= n `div` 2) a000290_list

a000161_list = map a000161 [0..]

-- Reinhard Zumkeller, Aug 16 2011

CROSSREFS

Cf. A002654, A001481, A002635, A025427, A025428, A063725, A025426, A000290, A010052.

Sequence in context: A056973 A107782 A086017 * A060398 A122855 A140727

Adjacent sequences:  A000158 A000159 A000160 * A000162 A000163 A000164

KEYWORD

nonn,core,easy,nice

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified July 30 03:23 EDT 2014. Contains 245051 sequences.