## Quick Reference

Any positive integer can be expressed as the sum of not more that four positive integers. Alternatively it can be expressed as the sum of squares of exactly four non-negative integers, see Lagrange theorem. So

1=1^{2}+0^{2}+0^{2}+0^{2},

5=2^{2}+1^{2}+0^{2}+0^{2},

15=3^{2}+2^{2}+1^{2}+1^{2}.

*Subjects:*
Mathematics.