OFFSET
1,1
COMMENTS
Found while writing a program to decompose integers as sums of four square numbers (following Lagrange's Four-Square Theorem).
Question: does the sum of the reciprocals of the numbers in this sequence converge? - J. Lowell, May 03 2014
Answer: this series is divergent. - Thomas Ordowski, May 22 2016
Numbers n such that A053610(n) > 4. - Thomas Ordowski, May 22 2016
LINKS
Enrique Pérez Herrero, Table of n, a(n) for n = 1..10000
E. J. Ionascu, Equilateral triangles in Z^4, arXiv:1209.0147 [math.NT], 2012-2013.
Eric Weisstein's World of Mathematics, Lagrange's Four-Square Theorem
Wikipedia, Lagrange's Four-Square Theorem
EXAMPLE
23 is the first number that cannot be decomposed because 16+4+1+1 falls short by one.
MATHEMATICA
f1[x_]:=Floor[Sqrt[x]];
f2[x_]:=Floor[Sqrt[x-f1[x]^2]];
f3[x_]:=Floor[Sqrt[x-f1[x]^2-f2[x]^2]];
f4[x_]:=Floor[Sqrt[x-f1[x]^2-f2[x]^2-f3[x]^2]];
Err[n_]:=n-(f1[n]^2+f2[n]^2+f3[n]^2+f4[n]^2);
Select[Table[n, {n, 0, 5000}], Err[#]!=0&] (* Enrique Pérez Herrero, Dec 19 2013 *)
PROG
(MATLAB) for n=1:312 a=n; i=1; while(i<5 & n~=0) j=1; while(j*j<=n) j=j+1; end; n=n-(j-1)*(j-1); i=i+1; end; if(n~=0) disp(a); end; end; % Luis F.B.A. Alexandre (lfbaa(AT)di.ubi.pt), Feb 08 2010
CROSSREFS
KEYWORD
nonn
AUTHOR
Luis F.B.A. Alexandre (lfbaa(AT)di.ubi.pt), Dec 31 2005
EXTENSIONS
Included terms where the final value of n is larger than 1. The fact that some terms might be missing was noted by Alonso del Arte on 2010-02-07. Luis F.B.A. Alexandre (lfbaa(AT)di.ubi.pt), Feb 08 2010
STATUS
approved